Fix DSO linking

Explicitely link against libm, to avoid linking problems:

CCLD   sbc/sbctester
/usr/bin/ld: sbc/sbctester.o: undefined reference to symbol 'sqrt@@GLIBC_2.2.5'
/usr/bin/ld: note: 'sqrt@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
/lib64/libm.so.6: could not read symbols: Invalid operation
1 file changed