build: require rsync

It seems to be required to build toolchain/kernel-headers.

Signed-off-by: Stijn Tintel <[email protected]>

README.md

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

include/prereq-build.mk

@@ -170,6 +170,9 @@ $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
 $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 	file --version 2>&1 | grep file))
 
+$(eval $(call SetupHostCommand,rsync,Please install 'rsync', \
+	rsync --version </dev/null))
+
 $(STAGING_DIR_HOST)/bin/mkhash: $(SCRIPT_DIR)/mkhash.c
 	mkdir -p $(dir $@)
 	$(CC) -O2 -I$(TOPDIR)/tools/include -o $@ $<