package/grpc: propagate reverse dependencies

It appears some grpc reverse dependencies aren't propagated properly.

Fixes:
WARNING: unmet direct dependencies detected for BR2_PACKAGE_GRPC
  Depends on [n]: BR2_INSTALL_LIBSTDCPP [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS [=y] && BR2_TOOLCHAIN_GCC_AT_LEAST_5 [=y] && !BR2_STATIC_LIBS [=n] && (BR2_TOOLCHAIN_HAS_SYNC_4 [=y] || BR2_TOOLCHAIN_HAS_ATOMIC [=y]) && BR2_PACKAGE_LIBABSEIL_CPP_ARCH_SUPPORTS [=n]
  Selected by [y]:
  - BR2_PACKAGE_FALCOSECURITY_LIBS [=y] && BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS [=y] && BR2_LINUX_KERNEL [=y] && BR2_INSTALL_LIBSTDCPP [=y] && BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && !BR2_STATIC_LIBS [=n] && BR2_TOOLCHAIN_USES_GLIBC [=y] && BR2_PACKAGE_LUAINTERPRETER_ABI_VERSION_5_1 [=y]

On BR2_PACKAGE_COLLECTD_GRPC, we take this opportunity to drop the
Config.in comment about the BR2_HOST_GCC_AT_LEAST_4_9, which was
dropped from BR2_PACKAGE_COLLECTD_GRPC in commit
8251d8c2 ("package/grpc: restrict
host-grpc to the absolute minimum")

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>