[Tizen] gyp: Move xmlsec detection to system.gyp.

This was the last remaining pkg-config call spread around random gyp
files. Move it to build/system.gyp with all the others and depend on the
"xmlsec" target in xwalk_application_common.gypi.
Merge pull request #2677 from rakuco/gyp-move-xmlsec-check-to-system.gyp

[Tizen] gyp: Move xmlsec detection to system.gyp.