Use call rather than batch scripts directly

[rodwiddowson]

Over Here @bagder noted (channelling Stefan Kanthak):

Apply the following changes to "winbuild/Makefile.vc" to fix build errors when AppLocker or SAFER alias Software Restriction Policies are in effect

--- curl-7.58.0/winbuild/Makefile.vc    2018-01-22 09:55
+++ curl-patch/winbuild/Makefile.vc
@@ -251,1 +251,1 @@
-    @gen_resp_file.bat $(LIBCURL_OBJS)
+    @CALL gen_resp_file.bat $(LIBCURL_OBJS)

--- curl-7.58.0/winbuild/Makefile.vc    2018-01-22 09:55
+++ curl-patch/winbuild/Makefile.vc
@@ -256,1 +256,1 @@
-    @gen_resp_file.bat $(CURL_OBJS)
+    @CALL gen_resp_file.bat $(CURL_OBJS)

Always use CALL to run batch scripts!

[MarcelRaad]

Looks good to me. Would you create a PR?