work around libzrtp warnings

This workaround makes the build work exactly as it did when libzrtp
was installed as a system library.  By default gcc ignores warnings
caused by system headers, so for the moment we'll continue to pretend
that libzrtp headers come from the system.

Makefile.am

@@ -149,7 +149,7 @@ endif
 if ENABLE_ZRTP
 CORE_CFLAGS += -I$(switch_srcdir)/libs/libzrtp/third_party/bgaes
 CORE_CFLAGS += -I$(switch_srcdir)/libs/libzrtp/third_party/bnlib
-CORE_CFLAGS += -I$(switch_srcdir)/libs/libzrtp/include
+CORE_CFLAGS += -isystem $(switch_srcdir)/libs/libzrtp/include
 libfreeswitch_la_LDFLAGS += -Llibs/libzrtp/third_party/bnlib -lbn -Llibs/libzrtp/projects/gnu/build -lzrtp
 CORE_LIBS += libs/libzrtp/projects/gnu/build/libzrtp.a
 endif