ruby: make ext module fiddle can compile success

ext module fiddle depend on libffi, in ruby source tree,
there is in internal libffi in case target platfrom don't
install libffi, but autotools.bbclass delete configure
under libffi and not run autoreconf to regenerate one.so
we met this error:
ruby-2.5.3/ext/fiddle/libffi-3.2.1/configure: No such file or directory

the fix is add depend and extra_oeconf to use libffi in the system

(From OE-Core rev: 3a0272bab546a4b47feab117713e32873419bbe7)

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/ruby/ruby.inc

@@ -14,7 +14,7 @@ LIC_FILES_CHKSUM = "\
     file://LEGAL;md5=23a79bb4c1a40f6cc9bcb6f4e7c39799 \
 "
 
-DEPENDS = "ruby-native zlib openssl tcl libyaml gdbm readline"
+DEPENDS = "ruby-native zlib openssl tcl libyaml gdbm readline libffi"
 DEPENDS_class-native = "openssl-native libyaml-native readline-native"
 
 SHRT_VER = "${@oe.utils.trim_version("${PV}", 2)}"

meta/recipes-devtools/ruby/ruby_2.5.3.bb

@@ -28,6 +28,7 @@ EXTRA_OECONF = "\
     --disable-dtrace \
     --enable-shared \
     --enable-load-relative \
+    --with-pkg-config=pkg-config \
 "
 
 do_install() {