Fix Mac build.

The Mac doesn't need fs_config (there's a `darwin: { enabled: false } }`
at the only caller), so stop trying to build it here rather than fix it
so it builds again.

Test: treehugger
Change-Id: I88d6b3a44b1ab823a419789d8545fcf10061079c

libcutils/Android.bp

@@ -170,10 +170,14 @@ cc_library {
         linux_bionic: {
             enabled: true,
         },
+        linux: {
+            srcs: [
+                "fs_config.cpp",
+            ],
+        },
         not_windows: {
             srcs: libcutils_nonwindows_sources + [
                 "ashmem-host.cpp",
-                "fs_config.cpp",
                 "trace-host.cpp",
             ],
         },
@@ -193,7 +197,6 @@ cc_library {
             srcs: libcutils_nonwindows_sources + [
                 "android_reboot.cpp",
                 "ashmem-dev.cpp",
-                "fs_config.cpp",
                 "klog.cpp",
                 "partition_utils.cpp",
                 "qtaguid.cpp",