components: Fix 'gn check' errors for user_prefs.

Tested on Linux with the following command lines:

$ gn gen out-gn/Debug
$ gn check out-gn/Debug //components/os_crypt/*
$ gn check out-gn/Debug //components/user_prefs/*

BUG=367595, 376000
TEST=see above
R=brettw@chromium.org,mnissler@chromium.org
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1213713002

Cr-Commit-Position: refs/heads/master@{#336414}

.gn

@@ -35,8 +35,10 @@ check_targets = [
   #"//components/*",  # Lots of errors.
   "//components/autofill/*",
   "//components/bookmarks/*",
+  "//components/os_crypt/*",
   "//components/pref_registry/*",
   "//components/undo/*",
+  "//components/user_prefs/*",
   "//components/webcrypto/*",
 
   #"//content/*",  # A whole lot of errors.

components/user_prefs/BUILD.gn

@@ -6,7 +6,7 @@ component("user_prefs") {
   sources = [
     "user_prefs.cc",
     "user_prefs.h",
-    "user_prefs/user_prefs_export.h",
+    "user_prefs_export.h",
   ]
 
   defines = [ "USER_PREFS_IMPLEMENTATION" ]