Use portable implementation for basename API

Use portable implementation for basename API

musl has removed the non-prototype declaration of basename from
string.h [1] which now results in build errors with clang-17+ compiler

Implement GNU basename behavior using strchr which is portable across libcs

Fixes
../git/tools/kmod.c:71:19: error: call to undeclared function 'basename'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
71 | "Commands:\n", basename(argv[0]));
| ^

[1] https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

Suggested-by: Rich Felker
Signed-off-by: Khem Raj <raj.khem@gmail.com>
[ Implement a basename() function in missing.h and ensure we always use
  the right include rather than having a separate gnu_basename() ]
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>