libpjsip: needs threads

Builds without thread support fail with the following error:

  ../src/pj/os_core_unix.c:45:21: fatal error: pthread.h: No such file or directory
   #include <pthread.h>
                       ^
  compilation terminated.

Fixes:
  http://autobuild.buildroot.org/results/744/744431e1b47d22222c4d0b48d5d20649dfeed929/
  http://autobuild.buildroot.org/results/a10/a10aca9139054502e055cb61f3925637fc515e12/

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpjsip/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_LIBPJSIP
 	bool "libpjsip"
 	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
 	  PJSIP is a free and open source multimedia communication
 	  library written in C language implementing standard based
@@ -8,5 +9,5 @@ config BR2_PACKAGE_LIBPJSIP
 
 	  http://www.pjsip.org
 
-comment "libpjsip needs a toolchain w/ C++"
-	depends on !BR2_INSTALL_LIBSTDCPP
+comment "libpjsip needs a toolchain w/ C++, threads"
+	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS