Make the legacy inline headers compile standalone.

It's useful to have the legacy inlines compile by themselves, both to
make header unification easier, and to ensure that the inline versions
match the regular declarations. Notably, this wasn't true for
sigismember, which took a const sigset_t* in the regular header, and
sigset_t* in the inline version.

Bug: http://b/28178111
Change-Id: Id8a3b7dcb1bfa61eed93c9fb50d3192744f8bef5
diff --git a/libc/include/android/legacy_stdlib_inlines.h b/libc/include/android/legacy_stdlib_inlines.h
index 58a2a9e..93554e5 100644
--- a/libc/include/android/legacy_stdlib_inlines.h
+++ b/libc/include/android/legacy_stdlib_inlines.h
@@ -29,8 +29,11 @@
 #ifndef _ANDROID_LEGACY_STDLIB_INLINES_H_
 #define _ANDROID_LEGACY_STDLIB_INLINES_H_
 
+#include <stdlib.h>
 #include <sys/cdefs.h>
 
+#if __ANDROID_API__ < 21
+
 __BEGIN_DECLS
 
 static __inline float strtof(const char *nptr, char **endptr) {
@@ -61,4 +64,5 @@
 
 __END_DECLS
 
+#endif
 #endif /* _ANDROID_LEGACY_STDLIB_INLINES_H_ */