include pgmspace.h for PSTR definition

Author: nerdralph

src/pu_print.h

@@ -1,5 +1,6 @@
 // picoUART print functions
 #pragma once
+#include <avr/pgmspace.h>
 
 extern "C" void prints(const char* s);
 extern "C" void prints_P(const char* s);