Update BUILDING.md - add dependency libseccomp-dev

I got these errors when I try to build containerd:

```
go build "-mod=vendor" "-buildmode=pie"  -tags "seccomp selinux apparmor" -ldflags "-X main.gitCommit="a220b9c6cc100f71ddc67152a12fc04622313dca" -X main.version=1.0.0-rc92+dev " -o runc .
Package libseccomp was not found in the pkg-config search path.
Perhaps you should add the directory containing `libseccomp.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libseccomp' found
Package libseccomp was not found in the pkg-config search path.
Perhaps you should add the directory containing `libseccomp.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libseccomp' found
pkg-config: exit status 1
```

Solved this problem by installing one extra dependency `libseccomp-dev`. I am using Ubuntu 20.04 and everything works well!

See also: opencontainers/runc#1032

Signed-off-by: Jun Lin Chen <webmaster@mc256.com>

BUILDING.md

@@ -38,11 +38,11 @@ $ sudo unzip protoc-3.11.4-linux-x86_64.zip -d /usr/local
 ```
 
 `containerd` uses [Btrfs](https://en.wikipedia.org/wiki/Btrfs) it means that you
-need to satisfy this dependencies in your system:
+need to satisfy these dependencies in your system:
 
 * CentOS/Fedora: `yum install btrfs-progs-devel`
 * Debian/Ubuntu: `apt-get install btrfs-tools`
-	* Debian Buster/Ubuntu 19.10:
+	* Debian Buster/Ubuntu 19.10/ Ubuntu 20.04:
 	    `apt-get install btrfs-progs libbtrfs-dev`
 
 At this point you are ready to build `containerd` yourself!
@@ -55,6 +55,12 @@ the system, sometimes it is necessary to build runc directly when working with
 container runtime development. You can skip this step if you already have the
 correct version of `runc` installed.
 
+`runc` requires `libseccomp`. You may need to install the missing dependencies:
+
+* CentOS/Fedora: `yum install libseccomp libseccomp-devel`
+* Debian/Ubuntu: `apt-get install libseccomp libseccomp-dev`
+
+
 For the quick and dirty installation, you can use the following:
 
     go get github.com/opencontainers/runc