minijail: Create a pkgconfig file for libminijail.

(cherry-picked from
 https://chromium.googlesource.com/a/chromiumos/platform2 at
 50fb55a05efdd0b31dc8dcbb53591bfff68f8703)

This allows us to change the import path of libminijail.h easily.

BUG=None
TEST=`emerge-gizmo apmanager attestation cromo cros-disks debugd
libchromeos lorgnette minijail tpm_manager trunks webserver` works.

CQ-DEPEND=CL:293580

Change-Id: I96a98d9f268fab518c2cc34c3d2ce2c5ae07788b
Reviewed-on: https://chromium-review.googlesource.com/293483
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/libchromeos.gypi b/libchromeos.gypi
index 03661cf..cfc7d46 100644
--- a/libchromeos.gypi
+++ b/libchromeos.gypi
@@ -183,9 +183,11 @@
     {
       'target_name': 'libchromeos-minijail-<(libbase_ver)',
       'type': 'shared_library',
-      'libraries': [
-        '-lminijail',
-      ],
+      'variables': {
+        'deps': [
+          'libminijail',
+        ],
+      },
       'cflags': [
         '-fvisibility=default',
       ],