Makefile.am: Link weston_multi_resource against rt library

Makefile.am: Link weston_multi_resource against rt library

This fixes this build failure:

/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.3/../../../../x86_64-unknown-linux-gnu/bin/ld:
clients/weston_multi_resource-multi-resource.o: undefined reference to
symbol 'clock_gettime@@GLIBC_2.2.5'
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.3/../../../../x86_64-unknown-linux-gnu/bin/ld:
note: 'clock_gettime@@GLIBC_2.2.5' is defined in DSO /lib64/librt.so.1
so try adding it to the linker command line
/lib64/librt.so.1: could not read symbols: Invalid operation