Makefile.vc: use /MANIFEST:EMBED
Makefile.vc: use /MANIFEST:EMBED

this removes the mt.exe step and may fix some build failures if the
executable is locked after linking, e.g., by an antivirus program

Change-Id: I9a8184f3aaecb62a133a13d9ad3383667bc93fd3

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp