libkmod: move function to the only file using it

libkmod: move function to the only file using it

If we don't have --gc-sections support, linking kmod fails:
libkmod/.libs/libkmod-util.a(libkmod-util.o): In function 'underscores':
libkmod/libkmod-util.c:117: undefined reference to 'kmod_log'

This is because libkmod-util.la uses kmod_log(), that is in libkmod.la.
Move the function so we don't have a dependency loop while building the
libraries and it works with compilers with no support for --gc-sections.