build: Ship install-sh in the tarball

Fixes the problem where configure from the tarball would report missing
files:

$ ./configure
configure: error: cannot find install-sh, install.sh, or shtool in bin

NOTE: This is a candidate for the 9.0 branch.

Makefile.am

 EXTRA_FILES = \
 	aclocal.m4					\
 	configure					\
+	bin/install-sh					\
 	src/glsl/glsl_parser.cc				\
 	src/glsl/glsl_parser.h				\
 	src/glsl/glsl_lexer.cc				\