strerror_r updates from build 3769769.

This is a partial https://android-review.googlesource.com/c/343952/.

We shipped this in r14, but r15 beta 1 is shipping the O oreview
headers, which is older than this change.

Test: ./validate.py
Bug: android/ndk#324
Change-Id: Ib301aba158f014d4fe2bf49aa7f2c1044c582f31
(cherry picked from commit 4e12436cc5af230b083e07ef3bea1d8ef1af1cf0)

platform/sysroot/usr/include/string.h

@@ -116,12 +116,8 @@ char* strerror(int);
 char* strerror_l(int, locale_t) __INTRODUCED_IN(23);
 #endif /* __ANDROID_API__ >= 23 */
 
-#if defined(__USE_GNU)
-
-#if __ANDROID_API__ >= 23
+#if defined(__USE_GNU) && __ANDROID_API__ >= 23
 char* strerror_r(int, char*, size_t) __RENAME(__gnu_strerror_r) __INTRODUCED_IN(23);
-#endif /* __ANDROID_API__ >= 23 */
-
 #else /* POSIX */
 int strerror_r(int, char*, size_t);
 #endif