Fix libm linking

Otherwise we get:
/usr/bin/ld: gnome-mouse-properties.o: undefined reference to symbol 'floor@@GLIBC_2.0'
/usr/bin/ld: note: 'floor@@GLIBC_2.0' is defined in DSO /lib/libm.so.6 so try adding it to the linker command line
/lib/libm.so.6: could not read symbols: Invalid operation

https://bugzilla.gnome.org/show_bug.cgi?id=609570