Resolve implicit declaration of function 'getpid'

gimp-debug.c:139:31: warning: implicit declaration of function 'getpid' [-Wimplicit-function-declaration] 139 | pid_t opid = getpid (); and use an undeprecated function https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-getpid?view=vs-2019 https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-getcurrentprocessid