tools/libtool: add missing dependency on m4

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 42322

tools/Makefile

@@ -44,7 +44,7 @@ tools-$(CONFIG_USE_SPARSE) += sparse
 $(curdir)/bison/compile := $(curdir)/flex/install
 $(curdir)/flex/compile := $(curdir)/automake/install
 $(curdir)/pkg-config/compile := $(curdir)/sed/install
-$(curdir)/libtool/compile := $(curdir)/sed/install
+$(curdir)/libtool/compile := $(curdir)/sed/install $(curdir)/m4/install
 $(curdir)/squashfs/compile := $(curdir)/lzma-old/install
 $(curdir)/squashfs4/compile := $(curdir)/xz/install
 $(curdir)/quilt/compile := $(curdir)/sed/install $(curdir)/autoconf/install $(curdir)/findutils/install