Issue # 80 - Fix up gfx/2d/DataSurfaceHelpers

De-unified build requires <string.h> instead of <cstring> to
prevent stdlib confusion.

gfx/2d/moz.build

@@ -141,15 +141,11 @@ elif CONFIG['CPU_ARCH'].startswith('mips'):
             'convolverLS3.cpp',
         ]
 
-# Won't build non-unified with a build error in iosfwd `strlen` (??)
-UNIFIED_SOURCES += [
-    'DataSourceSurface.cpp',
-    'DataSurfaceHelpers.cpp',
-]
-
 SOURCES += [
     'BezierUtils.cpp',
     'Blur.cpp',
+    'DataSourceSurface.cpp',
+    'DataSurfaceHelpers.cpp',
     'DrawEventRecorder.cpp',
     'DrawingJob.cpp',
     'DrawTarget.cpp',