Fix clang -Wtypedef-redefinition

Fixup for commit 77c184df7c

include/mysql/psi/psi_base.h

@@ -164,6 +164,13 @@ extern "C" {
 
 /** @} */
 
+/**
+  Instrumented memory key.
+  To instrument memory, a memory key must be obtained using @c register_memory.
+  Using a zero key always disable the instrumentation.
+*/
+typedef unsigned int PSI_memory_key;
+
 #ifdef __cplusplus
 }
 #endif