cgen: fix generic_fn_name generating incorrect names for C structs

vlib/v/gen/c/cgen.v

@@ -1271,7 +1271,8 @@ fn (mut g Gen) generic_fn_name(types []ast.Type, before string) string {
 	// `foo[int]()` => `foo_T_int()`
 	mut name := before + '_T'
 	for typ in types {
-		name += '_' + strings.repeat_string('__ptr__', typ.nr_muls()) + g.styp(typ.set_nr_muls(0))
+		name += '_' + strings.repeat_string('__ptr__', typ.nr_muls()) +
+			g.styp(typ.set_nr_muls(0)).replace(' ', '_')
 	}
 	return name
 }

vlib/v/tests/generics/foo.h

@@ -0,0 +1,9 @@
+// foo.h
+#ifndef TEST_H
+#define TEST_H
+
+struct foo {
+  int a;
+};
+
+#endif

vlib/v/tests/generics/generic_struct_cstruct_test.v

@@ -0,0 +1,14 @@
+#insert "@VMODROOT/foo.h"
+
+struct C.foo {
+	a int
+}
+
+fn ref[T](x T) &T {
+	return &x
+}
+
+fn test_main() {
+	a := C.foo{}
+	b := ref(a)
+}