Quoted CMAKE_C_FLAGS to fix build issue

The missing quotes caused a linebreak in CMAKE_C_FLAGS. make then ran two commands

cc <other flags>
-fvisibility=hidden <source>

causing a broken build:

Scanning dependencies of target libbladerf_sharedScanning dependencies of target libbladerf_shared
[  3%] Building C object libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/src/backend.c.o
cc: fatal error: no input files
compilation terminated.
/bin/sh: -fvisibility=hidden: command not found
make[2]: *** [libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/src/backend.c.o] Error 127
make[1]: *** [libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/all] Error 2
make: *** [all] Error 2
==> ERROR: A failure occurred in build().
    Aborting...

[  3%] Building C object libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/src/backend.c.o
cc: fatal error: no input files
compilation terminated.
/bin/sh: -fvisibility=hidden: command not found
make[2]: *** [libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/src/backend.c.o] Error 127
make[1]: *** [libraries/libbladeRF/CMakeFiles/libbladerf_shared.dir/all] Error 2
make: *** [all] Error 2
==> ERROR: A failure occurred in build().
    Aborting...