Hide which warning when fd isn't installed

On some systems, if `fd` isn't installed, this call to `which` will
result in a message like "which: no fd in <your PATH printed here>".
This hides that warning message.

common.mk

@@ -65,7 +65,7 @@ include $(base_dir)/tools/torture.mk
 #########################################################################################
 # Returns a list of files in directory $1 with file extension $2.
 # If available, use 'fd' to find the list of files, which is faster than 'find'.
-ifeq ($(shell which fd),)
+ifeq ($(shell which fd 2> /dev/null),)
 	lookup_srcs = $(shell find -L $(1)/ -name target -prune -o -iname "*.$(2)" -print 2> /dev/null)
 else
 	lookup_srcs = $(shell fd -L ".*\.$(2)" $(1))