mingw: Don't implement buildImportLib in only-c mode

src/mingw.zig

@@ -274,6 +274,7 @@ fn add_cc_args(
 }
 
 pub fn buildImportLib(comp: *Compilation, lib_name: []const u8) !void {
+    if (build_options.only_core_functionality) @panic("building import libs not included in core functionality");
     var arena_allocator = std.heap.ArenaAllocator.init(comp.gpa);
     defer arena_allocator.deinit();
     const arena = arena_allocator.allocator();