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

* Exclude `derb.cpp` as well as `derb.c` from Node builds
  (file was renamed in 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: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: #3066
Fixes: #3065

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': [