build: Restore OIIO_AVX512ER_ENABLED preprocessor symbol (#4735)

This useless symbol was removed in PR #4724, but maybe downstream things
used it and will break simply because it was removed, even though it
never would have been nonzero. It should not have been completely
removed except at a compatibility-breaking version boundary.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/include/OpenImageIO/simd.h

@@ -213,6 +213,7 @@
 #  define OIIO_AVX512CD_ENABLED 0
 #  define OIIO_AVX512BW_ENABLED 0
 #endif
+#define OIIO_AVX512ER_ENABLED 0 /* DEPRECATED(3.1) */
 
 #if defined(__FMA__)
 #  define OIIO_FMA_ENABLED 1