Silence unused-const-variable warnings in zstd.h on some gcc versions

Fixes bug 26272; bugfix on 0.3.1.1-alpha.

changes/bug26272

@@ -0,0 +1,3 @@
+  o Minor bugfixes (compilation):
+    - Silence unused-const-variable warnings in zstd.h on some gcc versions.
+      Fixes bug 26272; bugfix on 0.3.1.1-alpha.

src/common/compress_zstd.c

@@ -19,7 +19,9 @@
 #include "compress_zstd.h"
 
 #ifdef HAVE_ZSTD
+DISABLE_GCC_WARNING(unused-const-variable)
 #include <zstd.h>
+ENABLE_GCC_WARNING(unused-const-variable)
 #endif
 
 /** Total number of bytes allocated for Zstandard state. */