Adjust .gitignore for moved file

[l0rinc]

Split out of bitcoin#30454

Removed bitcoin-config.h.in from .gitignore since it's not generated anymore

[hebasto]

Adjust .gitignore entry for bitcoin-config.h.in, moved in https://github.com/bitcoin/bitcoin/blob/c360837ca5c91c9878ae8088bb5482e96fd87c96/test/lint/test_runner/src/main.rs#L321

When using Autotools, bitcoin-config.h.in is generated:

bitcoin/src/Makefile.am

Lines 1039 to 1040 in 818a3c0

	$(top_srcdir)/$(subdir)/config/bitcoin-config.h.in: $(am__configure_deps)
	$(AM_V_at)$(MAKE) -C $(top_srcdir) $(subdir)/config/bitcoin-config.h.in

However, bitcoin-config.h.in is a part of the source code in the cmake-staging branch and shouldn't be ignored by git.

[l0rinc]

Had a suspicion that was the case - updated to remove that line. If you don't think that's the correct way to solve it, feel free to close the PR.