remove duplicate 'mime.h' in Makefile. GNU cp complained about it. :-)

src/Makefile

@@ -98,7 +98,7 @@ install_headers: $(TARGET)
 	mkdir -p $(PREFIX)/include/pEp
 	cp pEpEngine.h keymanagement.h message_api.h dynamic_api.h stringlist.h \
 	   timestamp.h identity_list.h bloblist.h stringpair.h message.h mime.h \
-	   cryptotech.h sync_api.h blacklist.h pEp_string.h openpgp_compat.h mime.h \
+	   cryptotech.h sync_api.h blacklist.h pEp_string.h openpgp_compat.h \
 	   labeled_int_list.h key_reset.h base64.h sync_codec.h distribution_codec.h \
 	   status_to_string.h aux_mime_msg.h keyreset_command.h platform.h platform_unix.h ../asn.1/*.h \
  	   $(PREFIX)/include/pEp/