postgresql: enable staging installation

Certain packages need the headers/libraries from Postgresql to build
properly.

[Thomas: slightly reword the commit message.]

Signed-off-by: Nathaniel Roach <nroach44@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/postgresql/postgresql.mk

@@ -10,6 +10,8 @@ POSTGRESQL_SITE = http://ftp.postgresql.org/pub/source/v$(POSTGRESQL_VERSION)
 POSTGRESQL_LICENSE = PostgreSQL
 POSTGRESQL_LICENSE_FILES = COPYRIGHT
 
+POSTGRESQL_INSTALL_STAGING = YES
+
 ifneq ($(BR2_TOOLCHAIN_HAS_THREADS),y)
 	POSTGRESQL_CONF_OPT += --disable-thread-safety
 endif