package/libcgroup: needs threads

threads are mandatory since the addition of the package in commit and
https://github.com/libcgroup/libcgroup/commit/99ccdf671e9df5b13f646e1487393d562aa7fa62
resulting in the following build failure since commit
9d58b525fcfd0d92fcdd3c66d7858213c4b139c6 which enables build on uclibc
and musl:

In file included from config.c:30:
./libcgroup-internal.h:29:10: fatal error: pthread.h: No such file or directory
   29 | #include <pthread.h>
      |          ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/87c4454aee97b6955f461651c41c4186a8838c4d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libcgroup/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_LIBCGROUP
 	bool "libcgroup"
 	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	select BR2_PACKAGE_MUSL_FTS if !BR2_TOOLCHAIN_USES_GLIBC
 	help
 	  libcgroup is a library that abstracts the control
@@ -17,5 +18,5 @@ config BR2_PACKAGE_LIBCGROUP_TOOLS
 
 endif
 
-comment "libcgroup needs a toolchain w/ C++"
-	depends on !BR2_INSTALL_LIBSTDCPP
+comment "libcgroup needs a toolchain w/ C++, threads"
+	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS