Mark pkg-config dependency as development instead of runtime

Fixes ports that depend on ruby-rmagick, such as ruby-gruff.

Issue reported by naddy@. I missed committing this patch when
updating rmagick to 5.1.0.

graphics/ruby-rmagick/Makefile

@@ -2,6 +2,7 @@ COMMENT =	Ruby interface to ImageMagick
 
 DISTNAME =	rmagick-5.1.0
 CATEGORIES =	graphics
+REVISION =	0
 
 HOMEPAGE =	https://github.com/rmagick/rmagick
 

graphics/ruby-rmagick/patches/patch-_metadata

@@ -0,0 +1,14 @@
+Don't require pkg-config gem at runtime, as it is only used for building.
+
+Index: .metadata
+--- .metadata.orig
++++ .metadata
+@@ -20,7 +20,7 @@ dependencies:
+     - - "~>"
+       - !ruby/object:Gem::Version
+         version: '1.4'
+-  type: :runtime
++  type: :development
+   prerelease: false
+   version_requirements: !ruby/object:Gem::Requirement
+     requirements: