libcutils: Use proper private header to access C library's <time.h> extension

The strftime_tz() function is not a standard C library function and should not
appear in <time.h>. Instead, it is being moved to <bionic_time.h>, a private
header only available to platform libraries.

Change-Id: I4dc18939b9380725dfaa20c9e417296363bc52f4
diff --git a/include/cutils/tztime.h b/include/cutils/tztime.h
index cf103ca..36ac25d 100644
--- a/include/cutils/tztime.h
+++ b/include/cutils/tztime.h
@@ -26,8 +26,14 @@
 time_t mktime_tz(struct tm * const tmp, char const * tz);
 void localtime_tz(const time_t * const timep, struct tm * tmp, const char* tz);
 
-#ifndef HAVE_ANDROID_OS
-/* the following is defined in <time.h> in Bionic */
+#ifdef HAVE_ANDROID_OS
+
+/* the following is defined in the Bionic C library on Android, but the
+ * declarations are only available through a platform-private header
+ */
+#include <bionic_time.h>
+
+#else /* !HAVE_ANDROID_OS */
 
 struct strftime_locale {
     const char *mon[12];    /* short names */