gnu: icu4c: Explicitly add patch flags in replacement package.

This is to prevent a build failure when this gets merged to 'core-updates'
which does not have any patch-flags to inherit.

* gnu/packages/icu4c.scm (icu4c/fixed)[source](patch-flags): New field.

gnu/packages/icu4c.scm

@@ -111,6 +111,7 @@ C/C++ part.")
     (inherit icu4c)
     (source (origin
               (inherit (package-source icu4c))
+              (patch-flags '("-p2"))
               (patches (append
                          (origin-patches (package-source icu4c))
                          (search-patches