libcutils: convert fs_config.cpp

C++ify fs_config.c

Test: gTest libcutils-test
Bug: 37703469
Change-Id: Id48ae22f203ed923942257575296c69b32345ae6
diff --git a/libcutils/Android.bp b/libcutils/Android.bp
index 58170ec..245deb1 100644
--- a/libcutils/Android.bp
+++ b/libcutils/Android.bp
@@ -53,7 +53,7 @@
     host_supported: true,
     srcs: [
         "config_utils.c",
-        "fs_config.c",
+        "fs_config.cpp",
         "canned_fs_config.c",
         "hashmap.c",
         "iosched_policy.c",
@@ -94,6 +94,9 @@
             shared: {
                 enabled: false,
             },
+            cflags: [
+                "-D_GNU_SOURCE",
+            ],
         },
 
         android: {