kbuild: don't hardcode depmod path

depmod is not guaranteed to be in /sbin, just let make look for
it in the path like all the other invoked programs
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -450,7 +450,7 @@ LEX		= flex
 YACC		= bison
 AWK		= awk
 INSTALLKERNEL  := installkernel
-DEPMOD		= /sbin/depmod
+DEPMOD		= depmod
 PERL		= perl
 PYTHON		= python
 PYTHON3		= python3