package/gstreamer1/gst1-devtools: install to staging

package/gstreamer1/gst1-devtools: install to staging

This will fix the following build failure with
gstreamer1-editing-services which optionally depends on gst1-devtools
since commit 08a213c1a532de783a7cedaa29088ffb736827bd:

Run-time dependency gst-validate-1.0 found: NO (tried pkgconfig and cmake)
Looking for a fallback subproject for the dependency gst-validate-1.0

../output-1/build/gstreamer1-editing-services-1.18.0/meson.build:101:0: ERROR: Subproject directory not found and gst-devtools.wrap file not found

Fixes:
 - http://autobuild.buildroot.org/results/e9d96a63861737d82734559db5e5ffbf5996ffde

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>