package/libglvnd: needs threads

Fix the following build failure raised since the addition of the package
in commit 0378e2e5d9ce9d5676975dfbff501360094fdc1a:

In file included from ../src/util/glvnd_pthread.c:39:
../src/util/glvnd_pthread.h:33:10: fatal error: pthread.h: No such file or directory
   33 | #include <pthread.h>
      |          ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/63624bafc6f7a225d4db0667df977fd6141561aa

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/libglvnd/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_LIBGLVND
 	bool "libglvnd"
 	depends on BR2_TOOLCHAIN_HAS_SYNC_4 # __sync_*_4 intrisics
 	depends on !BR2_STATIC_LIBS # dlfcn.h
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
 	  The GL Vendor-Neutral Dispatch library
 
@@ -45,6 +46,6 @@ config BR2_PACKAGE_PROVIDES_LIBGLES
 
 endif
 
-comment "libglvnd needs a toolchain w/ dynamic library"
+comment "libglvnd needs a toolchain w/ dynamic library, threads"
 	depends on BR2_TOOLCHAIN_HAS_SYNC_4
-	depends on BR2_STATIC_LIBS
+	depends on BR2_STATIC_LIBS || !BR2_TOOLCHAIN_HAS_THREADS