puzzles: Ensure to link against libm for math functions
Without this we can see failures like:
| make: *** [samegame] Error 1
| /media/build1/poky/build/tmp/sysroots/x86_64-linux/usr/libexec/i586-poky-linux/gcc/i586-poky-linux/4.6.3/ld: inertia.o: undefined reference to symbol 'sqrt@@GLIBC_2.0'
| /media/build1/poky/build/tmp/sysroots/x86_64-linux/usr/libexec/i586-poky-linux/gcc/i586-poky-linux/4.6.3/ld: note: 'sqrt@@GLIBC_2.0' is defined in DSO /media/build1/poky/build/tmp/sysroots/qemux86/lib/libm.so.6 so try adding it to the linker command line
| /media/build1/poky/build/tmp/sysroots/qemux86/lib/libm.so.6: could not read symbols: Invalid operation
| collect2: ld returned 1 exit status
| make: *** [inertia] Error 1
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
