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>

package/libcpprestsdk/libcpprestsdk.mk

@@ -9,6 +9,7 @@ LIBCPPRESTSDK_SITE = $(call github,Microsoft,cpprestsdk,v$(LIBCPPRESTSDK_VERSION
 LIBCPPRESTSDK_LICENSE = MIT
 LIBCPPRESTSDK_LICENSE_FILES = license.txt
 LIBCPPRESTSDK_SUBDIR = Release
+LIBCPPRESTSDK_INSTALL_STAGING = YES
 LIBCPPRESTSDK_DEPENDENCIES += host-pkgconf boost openssl zlib
 LIBCPPRESTSDK_CONF_OPTS = -DWERROR=OFF -DBUILD_SAMPLES=OFF