Fix example_null Makefile for Mingw (#2590)
diff --git a/examples/example_null/Makefile b/examples/example_null/Makefile
index edf0145..d4159e4 100644
--- a/examples/example_null/Makefile
+++ b/examples/example_null/Makefile
@@ -61,6 +61,7 @@
 	ECHO_MESSAGE = "MinGW"
 	ifneq ($(WITH_EXTRA_WARNINGS), 0)
 		CXXFLAGS += -Wextra -Wpedantic
+		LIBS += -limm32
 	endif
 	CFLAGS = $(CXXFLAGS)
 endif