presence_xml: define _DARWIN_C_SOURCE 1

- compile the module on darwin, workaround for strptime

modules_k/presence_xml/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 _DARWIN_C_SOURCE 1
 #else
 	#define _XOPEN_SOURCE_EXTENDED 1   /* solaris */
 #endif