presence_conference: fix compiler warnings #612

> CC (gcc) [M presence_conference.so]		pidf.o
> In file included from /usr/include/time.h:27:0,
>                  from pidf.c:49:
> /usr/include/features.h:148:3: warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]
>  # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"

(cherry picked from commit 3976224)

modules/presence_conference/pidf.c

@@ -41,6 +41,7 @@
 	#define _BSD_SOURCE 1				/* needed on linux to "fix" the effect
 										  of the above define on
 										  features.h/unistd.h syscall() */
+   #define _DEFAULT_SOURCE 1			/* _BSD_SOURCE is deprecated */
    #define _DARWIN_C_SOURCE 1
 #else
 	#define _XOPEN_SOURCE_EXTENDED 1   /* solaris */