backends: Include unistd.h to fix compilation on musl
Musl doesn't seem to include this by default so explicitly including it should fix compilation on Musl.
Tested with Clang 16/17 and GCC 14.
Error: src/backends/meta-fd-source.c:70:3: error: call to undeclared function 'close'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration] close (fd_source->poll_fd.fd); ^
Closes: #3078 (closed)