app_fax: SpanDSP headers do not use ast_malloc; ignore that.

Since Asterisk 14, app_fax did not compile at all because Asterisk
requires that not malloc but ast_malloc is used everywhere. However,
the system headers of SpanDSP use malloc. Because we cannot (and do
not need to) change system headers, let us ignore this.

ASTERISK-28848

Change-Id: I31f7a6b92a07032c5cef1c16b8901b107fe35546

apps/app_fax.c

@@ -20,6 +20,8 @@
 	<replacement>res_fax</replacement>
 ***/
 
+/* Needed for spandsp headers */
+#define ASTMM_LIBC ASTMM_IGNORE
 #include "asterisk.h"
 
 #include <string.h>