[cmake] Include GNUInstallDirs before using variables defined by it.

This fixes an odd problem with the regex when `CMAKE_INSTALL_LIBDIR` is not
defined:

`string sub-command REGEX, mode REPLACE: regex "$" matched an empty string.`

Fixes #83802

llvm/cmake/modules/Findzstd.cmake

@@ -34,6 +34,7 @@ if(zstd_FOUND)
   elseif (NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
     if(MSVC)
+      include(GNUInstallDirs) # For CMAKE_INSTALL_LIBDIR and friends.
       # IMPORTED_LOCATION is the path to the DLL and IMPORTED_IMPLIB is the "library".
       get_filename_component(zstd_DIRNAME "${zstd_LIBRARY}" DIRECTORY)
       string(REGEX REPLACE "${CMAKE_INSTALL_LIBDIR}$" "${CMAKE_INSTALL_BINDIR}" zstd_DIRNAME "${zstd_DIRNAME}")