include string.h before poison pragma to avoid build issues on CentOS 7 (#123)

(cherry picked from commit 5902d8e)

Author: jfgrimm

src/alloc-private.h

@@ -28,6 +28,7 @@
 #define wpe_alloc_private_h
 
 #include <stdlib.h>
+#include <string.h>
 
 #if defined(__has_attribute) && __has_attribute(noreturn)
 #define WPE_NORETURN __attribute__((noreturn))