dav1dplay: Add -lm for llround() support

Neither --buildtype=plain nor --buildtype=debug set -ffast-math, so llround() is kept as a function call and isn’t optimised out into cvttsd2siq (on amd64), thus requiring the math lib to be linked.

Note that even with -ffast-math, it isn’t guaranteed that a call to llround() will always be omitted (I have reproduced this on PowerPC), so this fix is correct even if we ever decide to enable -ffast-math in other build types.

Without this fix, dav1dplay fails to build since 493d2b91 when using e.g. --buildtype=plain:

/usr/bin/ld: /tmp/dav1dplay.Z7jG9b.ltrans0.ltrans.o: undefined reference to symbol 'llround@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib/libm.so.6: error adding symbols: DSO missing from command line