Fix building of libicu port after recent llvm change. NFC (#24024)

Author: sbc100

tools/ports/icu.py

@@ -41,6 +41,7 @@ def build_lib(lib_output, lib_src, other_includes, build_flags):
         '-Wno-deprecated-declarations',
         '-Wno-array-compare',
         '-Wno-unknown-warning-option',
+        '-Wno-unnecessary-virtual-specifier',
         # usage of 'using namespace icu' is deprecated: icu v61
         '-DU_USING_ICU_NAMESPACE=0',
         # make explicit inclusion of utf header: ref utf.h