Makefile: sort inputs to linker so that build is reproducible

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
diff --git a/src/Makefile b/src/Makefile
index 08389ef..0e11d2b 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -75,7 +75,7 @@
 COMPILE.c += $(BUILT_IN_COMPILE.c)
 endif
 
-wg: $(patsubst %.c,%.o,$(wildcard *.c))
+wg: $(sort $(patsubst %.c,%.o,$(wildcard *.c)))
 
 ifneq ($(V),1)
 clean: