winbuild: reduce command-line length by dropping whitespace

winbuild: reduce command-line length by dropping whitespace

Keep the `@for %%i in [...]` lines within limits by stripping whitespace
from the input `.c` source lists read from `Makefile.inc`. To avoid this
error after adding a new `.c` source:
```
configuration name: libcurl-vc14-x64-release-dll-ssl-dll-ipv6-sspi
NMAKE : fatal error U1095: expanded command line 'for %i in (altsvc.obj            amigaos.obj
           asyn-ares.obj         asyn-thread.obj       base64.obj            bufq.obj
              bufref.obj            cf-h1-proxy.obj       cf-h2-proxy.obj       cf-haproxy.obj [...]
  vssh/wolfssh.obj) do @echo ..\builds\libcurl-vc14-x64-release-dll-ssl-dll-ipv6-sspi-obj-lib/%i \
                   ' too long
Stop.
Command exited with code 2
```
Ref: https://ci.appveyor.com/project/curlorg/curl/builds/51605338/job/dqg6qtebtscb279g#L44

Reported-by: Stefan Eissing
Bug: https://github.com/curl/curl/pull/16508#issuecomment-2690443409
Fixes #16521
Closes #16528