Add a missing -lm to Makefile.

diff --git a/Makefile b/Makefile
index 1b49d7f..891f36e 100644
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@ HEADER_FILE=simple_audio.h
 all: libsimple_audio.so

 libsimple_audio.so:
-	$(CC) -fPIC -shared -lm -o $(SHARED_LIB) $(C_SOURCE)
+	$(CC) -fPIC -shared -lm -o $(SHARED_LIB) $(C_SOURCE) -lm

 example: libsimple_audio.so
 	$(CC) -o main main.c libsimple_audio.so