[cmakelists] Symbol visibility hidden (-fvisibility=hidden)

[CastagnaIT]

Description

User noticed weird behaviour under linux only
when you play a widevine video and after a clearkey video this last one dont works anymore resulting in various errors,
this is only an example but the problem is reproducible in several similar ways with different DRM's

the problem are the static variables, in particular there is a singleton, the service broker

inputstream.adaptive/src/SrvBroker.h

Lines 59 to 63 in 6800302

	static CSrvBroker* GetInstance()
	{
	static CSrvBroker instance;
	return &instance;
	}

then problem raise because shared libraries under linux (opened by kodi core with dlopen/dlclose) by default export static data in order to be shared/linked to all instances of the library, this cause that the second time that you play a video the service broker contains the data of the first video played and so cause any kind of weirdness, errors, etc...

this problem under Windows dont happens because by default symbols are internal (hidden)

there are different ways to "hide" symbols i chosen to add compiler parameter -fvisibility=hidden to apply it globally

side note:
im not sure because i have not time to test it on a LibreELEC device but there is possibility that could fix #454 about "Permanent failure" error, see comment #727 (comment), where seem that widevine library could have static variables

Motivation and context

fix #1704

How has this been tested?

builded on ubuntu then,

1. play widevine video
2. play clearkey video
3. the second video must be played without problem

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly