Revert "win32: libwebp has been bumped to version 5"

This reverts commit 18efb211576ca0e9fce2b8f4e00977a9f4ea2ace.

Our toolchain relies on an older version now that has version 4

cmake/modules/NSIS.template.in

@@ -443,7 +443,7 @@ Section "${APPLICATION_NAME}" SEC_APPLICATION
    File "${MING_BIN}\libcrypto-10.dll"
    File "${MING_BIN}\libssl-10.dll"
    File "${MING_BIN}\libstdc++-6.dll"
-   File "${MING_BIN}\libwebp-5.dll"
+   File "${MING_BIN}\libwebp-4.dll"
    File "${MING_BIN}\libxslt-1.dll"
    File "${MING_BIN}\libxml2-2.dll"
    File "${MING_BIN}\zlib1.dll"