Switch mkbootfs to C++.

Minimum changes just to get it to compile as C++, no real cleanup.

Change-Id: I8ff3fa35a07cdc9a6a246e79e33581e5d6598833
diff --git a/mkbootfs/Android.bp b/mkbootfs/Android.bp
index cd2a624..e0191f0 100644
--- a/mkbootfs/Android.bp
+++ b/mkbootfs/Android.bp
@@ -6,7 +6,7 @@
 
 cc_binary_host {
     name: "mkbootfs",
-    srcs: ["mkbootfs.c"],
+    srcs: ["mkbootfs.cpp"],
     cflags: ["-Werror"],
     static_libs: [
         "libbase",