CMake: use EXCLUDE_FROM_ALL when declaring Fetch for deflate library

Commit af9ef0e made a change in the CMake setup to switch from using
FetchContent_Populate() to FetchContent_MakeAvailable() to bring in
libdeflate (and Imath) as dependencies. Using FetchContent_MakeAvailable()
has the side effect that libdeflate is now being included as part of the
build, causing libraries and headers to be built and installed.

The intent with fetching the deflate source is solely to copy select
files into OpenEXRCore, so this change adds the EXCLUDE_FROM_ALL option
to the call to FetchContent_Declare() to prevent deflate from being
included in the build.

cmake/OpenEXRSetup.cmake

@@ -219,10 +219,13 @@ else()
     message(STATUS "libdeflate was not found, installing from ${OPENEXR_DEFLATE_REPO} (${OPENEXR_DEFLATE_TAG})")
   endif()
   include(FetchContent)
+  # Fetch deflate but exclude it from the "all" target.
+  # This prevents the library from being built.
   FetchContent_Declare(Deflate
     GIT_REPOSITORY "${OPENEXR_DEFLATE_REPO}"
     GIT_TAG "${OPENEXR_DEFLATE_TAG}"
     GIT_SHALLOW ON
+    EXCLUDE_FROM_ALL
     )
 
   FetchContent_GetProperties(Deflate)