Fix gcc 15.2.1 warning for discarding 'const' qualifier

[EddyGharib]

Description

Compiling with gcc version 15.2.1 generates the following warning

external/ros2_rcutils+/src/strdup.c: In function 'rcutils_strndup':
external/ros2_rcutils+/src/strdup.c:47:14: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
   47 |   char * p = memchr(str, '\0', max_length);
      |              ^~~~~~

This PR fixes that

Fixes # (issue)

Is this user-facing behavior change?

Did you use Generative AI?

Additional Information

[fujitatomoya]

Pulls: #545
Gist: https://gist.githubusercontent.com/fujitatomoya/8cf86a9def1a24cedfcc2c33f0264c43/raw/9570b4b0f68b58a34e1fbb9e7c2bd63fec6db4ab/ros2.repos
BUILD args: --packages-above-and-dependencies rcutils
TEST args: --packages-above rcutils
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18402

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[ahcorde]

Do you mind to target rolling ? then we can backport this to older distros