cgo: add package directory to header include paths

testdata/cgo/main.c

@@ -1,3 +1,5 @@
+#include "main.h"
+
 int fortytwo() {
 	return 42;
 }

testdata/cgo/main.go

@@ -2,8 +2,7 @@ package main
 
 /*
 int fortytwo(void);
-typedef short myint;
-int add(int a, int b);
+#include "main.h"
 */
 import "C"
 

testdata/cgo/main.h

@@ -0,0 +1,2 @@
+typedef short myint;
+int add(int a, int b);