singularity: added missing dependencies (#16430)

See:

https://sylabs.io/guides/3.5/user-guide/quick_start.html#install-system-dependencies

for pkg-config and libseccomp

var/spack/repos/builtin/packages/singularity/package.py

@@ -40,9 +40,12 @@ class Singularity(MakefilePackage):
 
     variant('suid', default=True, description='install SUID binary')
     variant('network', default=True, description='install network plugins')
+
+    depends_on('pkgconfig', type='build')
     depends_on('go')
     depends_on('libuuid')
     depends_on('libgpg-error')
+    depends_on('libseccomp')
     depends_on('squashfs', type='run')
     depends_on('git', when='@develop')  # mconfig uses it for version info
     depends_on('shadow', type='run', when='@3.3:')