autoPatchelfHook: Silence errors in isExecutable

The "maxx" package recursively runs isExecutable on a bunch of files and
since the change to use "readelf" instead of "file" a lot of errors like
this one are printed during build:

  readelf: Error: Not an ELF file - it has the wrong magic bytes at the
  start

While the isExecutable was never meant to be used outside of the
autoPatchelfHook, it's still a good idea to silence the errors because
whenever readelf fails, it clearly indicates that the file in question
is not a valid ELF file.

Signed-off-by: aszlig <aszlig@nix.build>
(cherry picked from commit b4526040a2)

pkgs/build-support/setup-hooks/auto-patchelf.sh

@@ -7,7 +7,7 @@ gatherLibraries() {
 addEnvHooks "$targetOffset" gatherLibraries
 
 isExecutable() {
-    readelf -h "$1" | grep -q '^ *Type: *EXEC\>'
+    readelf -h "$1" 2> /dev/null | grep -q '^ *Type: *EXEC\>'
 }
 
 # We cache dependencies so that we don't need to search through all of them on