Fix the following compile-time warning:

making all mod_avmd
make[7]: Entering directory '/home/sk/tmp/lede/build_dir/target-mips_24kc_musl-1.1.15/freeswitch-1.8.0/src/mod/applications/mod_avmd'
  CC       mod_avmd_la-mod_avmd.lo
mod_avmd.c: In function 'avmd_process_sample':
mod_avmd.c:49:19: error: implicit declaration of function '__isinf' [-Werror=implicit-function-declaration]
 #define ISINF(x) (__isinf(x))
                   ^
mod_avmd.c:2038:33: note: in expansion of macro 'ISINF'
         if (ISNAN(amplitude) || ISINF(amplitude)) {
                                 ^
cc1: all warnings being treated as errors
Makefile:682: recipe for target 'mod_avmd_la-mod_avmd.lo' failed

Fix this by adding the declaration for __isinf(), the same is done for
__isnan() already
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>