Fix implicit-declaration error on some systems

[lgbaldoni]

I'm seeing this on openSUSE Leap 15.1

lib/path.c: In function 'srn_get_executable_path':
lib/path.c:370:4: warning: implicit declaration of function 'realpath'; did you mean 'renameat'? [-Wimplicit-function-declaration]
    realpath(PROC_SELF_EXE, rawPathName);
    ^~~~~~~~
    renameat

[SilverRainZ]

Thank you!