madplay: fix compilation with GCC 15

[tk154]

📦 Package Details

Maintainer: @probonopd

Description:
Currently, madplay fails to build while using GCC 15:

In file included from ../../../staging_dir/toolchain-x86_64_gcc-15.2.0_musl/include/fortify/unistd.h:22,
                 from madplay.c:42:
staging_dir/toolchain-x86_64_gcc-15.2.0_musl/include/unistd.h:127:5: error: conflicting types for 'getopt'; have 'int(int,  char * const*, const char *)'
  127 | int getopt(int, char * const [], const char *);
      |     ^~~~~~
In file included from madplay.c:29:
getopt.h:147:12: note: previous declaration of 'getopt' with type 'int(void)'
  147 | extern int getopt ();
      |            ^~~~~~

This patch defines __GNU_LIBRARY__ for a proper getopt declaration when building with GCC 15.

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🧪 Run Testing Details

• OpenWrt Version: Snapshot
• OpenWrt Target/Subtarget: x86/64
• OpenWrt Device: Generic x86/64

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✅ Formalities

• I have reviewed the CONTRIBUTING.md file for detailed contributing guidelines.