temp/mesa-git: re-add dev() and define provides for -dev package (!924)

This fixes 1943ead268, which accidentally
removed the dev() function that generates the mesa-git-dev package..
causing mesa-dev to be pulled in in cases where mesa-dev is required
(and failing because mesa-dev is older)

[ci:skip-build]: already built successfully in CI

temp/mesa-git/APKBUILD

@@ -5,7 +5,7 @@
 # new Mesa stable release
 pkgname=mesa-git
 pkgver=0_git20200202
-pkgrel=1
+pkgrel=2
 _commit="0021f7dc307f4852955359adb5ac2b7667e6d4ac"
 pkgdesc="Mesa DRI OpenGL library (Git)"
 url="https://www.mesa3d.org"
@@ -170,6 +170,12 @@ package() {
 	sed -i s/-devel//g "$pkgdir"/usr/lib/pkgconfig/*.pc
 }
 
+dev() {
+	provides="mesa-dev"
+	replaces="mesa-dev"
+	default_dev
+}
+
 egl() {
 	pkgdesc="Mesa libEGL runtime libraries"
 	depends="$pkgname"