Add internal.c to unix Makefile's libopusurl.

This shouldn't really be necessary when static linking, but would
 be if someone wanted chang the Makefile to build shared libraries.

unix/Makefile

@@ -75,6 +75,7 @@ LIBOPUSFILE_CHEADERS = \
 internal.h \
 
 LIBOPUSURL_CSOURCES = \
+internal.c \
 http.c \
 
 ifneq ($(findstring mingw,${CC}),)