Restore compatibility with GNU Radio 3.7.12.0

[noc0lour]

From the discuss-gnuradio mailinglist:

[  1%] Building CXX object lib/CMakeFiles/gnuradio-fbmc.dir/fbmc_config.cc.o
/usr/local/src/gr-fbmc/lib/fbmc_config.cc: In member function ‘void
gr::fbmc::fbmc_config::gen_preamble_sym()’:
/usr/local/src/gr-fbmc/lib/fbmc_config.cc:277:7: error: ‘gr_uint16’ was not
declared in this scope
       gr_uint16 start_state = 0xACE1u; /* Any nonzero start start will
work. */
       ^
/usr/local/src/gr-fbmc/lib/fbmc_config.cc:278:18: error: expected ‘;’
before ‘lfsr’
       gr_uint16  lfsr = start_state;
                  ^
/usr/local/src/gr-fbmc/lib/fbmc_config.cc:285:17: error: ‘lfsr’ was not
declared in this scope
         bit = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 5)) & 1;
                 ^
lib/CMakeFiles/gnuradio-fbmc.dir/build.make:62: fallo en las instrucciones
para el objetivo 'lib/CMakeFiles/gnuradio-fbmc.dir/fbmc_config.cc.o'
make[2]: *** [lib/CMakeFiles/gnuradio-fbmc.dir/fbmc_config.cc.o] Error 1
CMakeFiles/Makefile2:137: fallo en las instrucciones para el objetivo
'lib/CMakeFiles/gnuradio-fbmc.dir/all'
make[1]: *** [lib/CMakeFiles/gnuradio-fbmc.dir/all] Error 2
Makefile:138: fallo en las instrucciones para el objetivo 'all'
make: *** [all] Error 2