Unbreak build with multiple make jobs

Without it python_curlopt.h might not be available at the right
time when using something like make -j4:

src/python.c:50:10: fatal error: 'python_curlopt.h' file not found
         ^~~~~~~~~~~~~~~~~~
1 error generated.

Signed-off-by: Tobias Kortkamp <t@tobik.me>

Makefile

@@ -203,6 +203,8 @@ uninstall:
 $(OBJDIR)/%.o: src/%.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
+src/python.c: $(PYTHON_CURLOPT)
+
 clean:
 	rm -f $(VERSION)
 	find . -type f -name \*.o -exec rm {} \;