Bug 1520310 - Disable SANDBOX_EXPORTS for the mingw-clang build as well r=bobowen

security/sandbox/moz.build

@@ -159,7 +159,7 @@ elif CONFIG['OS_ARCH'] == 'WINNT':
     for var in ('UNICODE', '_UNICODE', 'NS_NO_XPCOM',
                 '_CRT_RAND_S', 'CHROMIUM_SANDBOX_BUILD'):
         DEFINES[var] = True
-    if CONFIG['CC_TYPE'] != 'gcc':
+    if CONFIG['CC_TYPE'] not in ('gcc', 'clang'):
         DEFINES['SANDBOX_EXPORTS'] = True
 
     LOCAL_INCLUDES += ['/security/sandbox/chromium-shim']