package/pulseview: fix build with libglib >= 2.68

Fix the following build failure with libglib >= 2.68 raised since commit
c72524fb:

In file included from /tmp/instance-0/output-1/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/glib-2.0/glib/gatomic.h:31,
                 from /tmp/instance-0/output-1/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/glib-2.0/glib/gthread.h:32,
                 from /tmp/instance-0/output-1/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/glib-2.0/glib/gasyncqueue.h:32,
                 from /tmp/instance-0/output-1/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/glib-2.0/glib.h:32,
                 from /tmp/instance-0/output-1/host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/libsigrokdecode/libsigrokdecode.h:25,
                 from /tmp/instance-0/output-1/build/pulseview-0.4.2/pv/data/decode/annotation.cpp:21:
/tmp/instance-0/output-1/host/opt/ext-toolchain/mips64el-buildroot-linux-uclibc/include/c++/8.4.0/type_traits:56:3: error: template with C linkage
   template<typename _Tp, _Tp __v>
   ^~~~~~~~
/tmp/instance-0/output-1/build/pulseview-0.4.2/pv/data/decode/annotation.cpp:20:1: note: 'extern "C"' linkage started here
 extern "C" {
 ^~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/33cbaaeefd8637bac4ad0fbbfae7c369de4875d2



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