Add support for Gentoo to source-repositories

As mentioned in package-installs.yaml, git is a transitive dependency
for pbr in this element.  Add pkg-map for the Gentoo package.

Change-Id: I7f2fe1663152ea66b941594e86f1da93ddd21677
Closes-Bug: 1539278

elements/source-repositories/pkg-map

@@ -0,0 +1,10 @@
+{
+  "family": {
+    "gentoo": {
+      "git": "dev-vcs/git"
+    }
+  },
+  "default": {
+    "git": "git"
+  }
+}