add localmatcher.h to common shim headers

localmatcher.h was added in icu-68.1 and is required with:
https://crrev.com/741586ecfbe42908e66016eb6319a1cc0ffc379e

Bug: None
Change-Id: I5167cb3d6f35dd0c4d6268b46c876579f72656dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574858Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834304}

build/linux/unbundle/icu.gn

@@ -175,6 +175,7 @@ shim_headers("icuuc_shim") {
     "unicode/icudataver.h",
     "unicode/icuplug.h",
     "unicode/idna.h",
+    "unicode/localematcher.h",
     "unicode/localpointer.h",
     "unicode/locdspnm.h",
     "unicode/locid.h",