libweston,tools: Include libgen.h for basename signature

Latest musl has removed the declaration from string.h [1] as it only
implements POSIX version alone and string.h in glibc implements GNU
version of basename. This now results in compile errors on musl.

This might be a warning with older compilers but it is error with
Clang-17+ as it treats -Wimplicit-function-declaration as error

Switch the use in backlight_init function to use POSIX version

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



Signed-off-by: Khem Raj <raj.khem@gmail.com>