package/libcpprestsdk: install to staging

package/libcpprestsdk: install to staging

libcpprestsdk is a library so it makes sense to install it in
staging even if no current package is using it for now

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=11936

Signed-off-by: Hein Gustavsen <hein.gustavsen@miros-group.com>
[Retrieved (and slightly update commit message) from:
https://bugs.buildroot.org/show_bug.cgi?id=11936]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>