package/pistache: fix dependency on rapidjson

rapidjson has been added as a dependency of pistache as part of commit
2f3218567953185008ec862f814bebecb9d6ed82 ("package/pistache: bump
version & switch to meson build"), using a "depends on". However, as
it is a library dependency, a "select" is normally used in Buildroot.

rapidjson's only dependency is on C++ support, which is already a
dependency of pistache.

In addition, use = instead of += for the rapidjson dependency in the
.mk file, as it's an unconditional dependency.

Cc: Thomas Ruschival <thomas@ruschival.de>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 240aae4793022e4fe26ed7fa745d30e33a02fc4b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pistache/Config.in

@@ -4,9 +4,9 @@ config BR2_PACKAGE_PISTACHE
 	depends on BR2_USE_WCHAR
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	depends on BR2_INSTALL_LIBSTDCPP
-	depends on BR2_PACKAGE_RAPIDJSON
 	depends on !BR2_TOOLCHAIN_USES_UCLIBC
 	depends on !BR2_TOOLCHAIN_HAS_BINUTILS_BUG_27597
+	select BR2_PACKAGE_RAPIDJSON
 	help
 	  Pistache is a modern and elegant HTTP and REST framework
 	  for C++. It is entirely written in pure C++17 and provides

package/pistache/pistache.mk

@@ -10,7 +10,7 @@ PISTACHE_LICENSE = Apache-2.0
 PISTACHE_LICENSE_FILES = LICENSE
 
 PISTACHE_INSTALL_STAGING = YES
-PISTACHE_DEPENDENCIES += rapidjson
+PISTACHE_DEPENDENCIES = rapidjson
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 PISTACHE_DEPENDENCIES += openssl