build: don't build icu with -fno-rtti

ICU should be compiled with -frtti (and it sets that flag in its gyp
file) but it was also inheriting the -fno-rtti flag from common.gypi,
breaking the build on some systems.

Fixes: https://github.com/nodejs/node/issues/8867
PR-URL: https://github.com/nodejs/node/pull/8886
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

tools/icu/icu-generic.gyp

@@ -67,6 +67,7 @@
           [ 'os_posix == 1 and OS != "mac" and OS != "ios"', {
             'cflags': [ '-Wno-deprecated-declarations' ],
             'cflags_cc': [ '-frtti' ],
+            'cflags_cc!': [ '-fno-rtti' ],
           }],
           [ 'OS == "mac" or OS == "ios"', {
             'xcode_settings': {'GCC_ENABLE_CPP_RTTI': 'YES' },