Make <android/dlext.h> compile standalone.

Bug: http://b/28067717
Change-Id: If087e0c931c69bda34263ea8e0b2dc46d932b2af
diff --git a/libc/include/android/dlext.h b/libc/include/android/dlext.h
index d5ec386..6e32b68 100644
--- a/libc/include/android/dlext.h
+++ b/libc/include/android/dlext.h
@@ -17,6 +17,7 @@
 #ifndef __ANDROID_DLEXT_H__
 #define __ANDROID_DLEXT_H__
 
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/cdefs.h>