README: add info about required libseccomp version

Support for SCMP_FLTATR_CTL_TSYNC (required by qemu) in theory is in
2.2.0, but it is not enabled there yet. Requires libseccomp >= 2.3.0 to
actually have this flag functional.
See seccomp/libseccomp#20

(cherry picked from commit e762ebb)

README.md

@@ -19,7 +19,7 @@ Building
    - automake
    - edk2-tools
    - libtool
-   - libseccomp-devel
+   - libseccomp-devel >= 2.3.0
    - pixman-devel
    - xen-devel