package/pifmrds: always link with -lm

Commit 888546e5 wrongly removed linking
with -lm resulting in the following build failure:

/home/buildroot/autobuild/run/instance-3/output-1/host/bin/arm-linux-gnueabihf-gcc  -o pi_fm_rds rds.o waveforms.o pi_fm_rds.o fm_mpx.o control_pipe.o -L/home/buildroot/autobuild/run/instance-3/output-1/host/bin/../arm-buildroot-linux-gnueabihf/sysroot/usr/lib -lsndfile
/home/buildroot/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-linux-gnueabihf/7.3.1/../../../../arm-linux-gnueabihf/bin/ld: fm_mpx.o: undefined reference to symbol 'cos@@GLIBC_2.4'
/home/buildroot/autobuild/run/instance-3/output-1/host/arm-buildroot-linux-gnueabihf/sysroot/lib/libm.so.6: error adding symbols: DSO missing from command line

Fixes:
 - http://autobuild.buildroot.org/results/b2a6e6fd77bf9071ce9f75fed1811be9ffe5366d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>