build: add which command to build requirements

`which` utility is not shipped by default for example on recent Arch
Linux and then any steps relying on its presence fails, like for example
following Python3 prereq build check:

 $ python3 --version
 Python 3.9.1

 $ make
 /bin/sh: line 1: which: command not found
 ...
 Checking 'python3'... failed.

So make `which` utility host build requirement.

References: PR#3820 FS#3525
Signed-off-by: Petr Štetiar <ynezz@true.cz>

README.md

@@ -26,7 +26,7 @@ documentation.
 
 ```
 gcc-6+ binutils bzip2 flex python3 perl make4.1+ find grep diff unzip gawk getopt
-subversion libz-dev libc-dev rsync
+subversion libz-dev libc-dev rsync which
 ```
 
 ### Quickstart

include/prereq-build.mk

@@ -166,6 +166,9 @@ $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 $(eval $(call SetupHostCommand,rsync,Please install 'rsync', \
 	rsync --version </dev/null))
 
+$(eval $(call SetupHostCommand,which,Please install 'which', \
+	which which | grep which))
+
 $(STAGING_DIR_HOST)/bin/mkhash: $(SCRIPT_DIR)/mkhash.c
 	mkdir -p $(dir $@)
 	$(CC) -O2 -I$(TOPDIR)/tools/include -o $@ $<