Build C musl additions as C99.

This is how the upstream C is built for bionic proper, so it causes unfortunate surprises for the musl build to differ in this way (http://b/395885866).

Change-Id: Id8fb1bff11adfa383fbd7b18d1a0151ad51defa2
diff --git a/libc/Android.bp b/libc/Android.bp
index 7b902dc..b5ff680 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -2703,6 +2703,7 @@
     },
 }
 
+// TODO: add this directly to musl like libexecinfo and libb64?
 cc_library_host_static {
     name: "libfts",
     srcs: [
@@ -2715,6 +2716,7 @@
         "upstream-openbsd/android/include",
     ],
     cflags: [
+        "-std=gnu99",
         "-include openbsd-compat.h",
         "-Wno-unused-parameter",
     ],
@@ -2754,6 +2756,7 @@
         "upstream-openbsd/android/include",
     ],
     cflags: [
+        "-std=gnu99",
         "-include openbsd-compat.h",
     ],
     enabled: false,