build: Intl: avoid 'duplicate main()' on ICU 56

ICU 56 renamed derb.c to derb.cpp because of C++ yay.
This broke the exclusion of "derb.c" when building tools.

Solution is to add derb.c AND derb.cpp to exclusion.
We don't build the 'derb' tool, so it's fine to list the
excluded source twice.

Reviewed-By:
PR-URL:
Fixes: nodejs/node#3065

Author: srl295

tools/icu/icu-generic.gyp

@@ -7,7 +7,10 @@
 
 {
   'variables': {
-    'icu_src_derb': [ '../../deps/icu/source/tools/genrb/derb.c' ],
+    'icu_src_derb': [
+      '../../deps/icu/source/tools/genrb/derb.c',
+      '../../deps/icu/source/tools/genrb/derb.cpp'
+    ],
   },
   'includes': [ '../../icu_config.gypi' ],
   'targets': [