fixup! Bug 41459: WebRTC fails to build under mingw (Part 4)

Define _CRT_RAND_S as a compiler flag, to avoid problems due to unified
sources.

netwerk/sctp/src/moz.build

@@ -56,3 +56,5 @@ if CONFIG['OS_TARGET'] in ('Linux', 'Android'):
     DEFINES['_GNU_SOURCE'] = 1
 elif CONFIG['OS_TARGET'] == 'Darwin':
     DEFINES['__APPLE_USE_RFC_2292'] = 1
+elif CONFIG['OS_TARGET'] == 'WINNT':
+    DEFINES['_CRT_RAND_S'] = True