cov: define the build_database method on the code coverage plugin
cov: define the build_database method on the code coverage plugin

This method was claimed as implemented by that plugin, but never defined.
Provide an empty implementation as that plugin doesn't deal with the
database.

https://bugzilla.mozilla.org/show_bug.cgi?id=760158

pelmers/dxr,bozzmob/dxr,srenatus/dxr,jonasfj/dxr,jbradberry/dxr,pombredanne/dxr,srenatus/dxr,KiemVM/Mozilla--dxr,erikrose/dxr,bozzmob/dxr,nrc/dxr,nrc/dxr,pombredanne/dxr,pombredanne/dxr,pelmers/dxr,erikrose/dxr,kleintom/dxr,srenatus/dxr,nrc/dxr,pombredanne/dxr,gartung/dxr,jonasfj/dxr,kleintom/dxr,kleintom/dxr,pombredanne/dxr,jbradberry/dxr,KiemVM/Mozilla--dxr,jonasfj/dxr,pelmers/dxr,jay-z007/dxr,gartung/dxr,gartung/dxr,erikrose/dxr,gartung/dxr,bozzmob/dxr,nrc/dxr,pelmers/dxr,jonasfj/dxr,jay-z007/dxr,kleintom/dxr,nrc/dxr,KiemVM/Mozilla--dxr,pombredanne/dxr,jbradberry/dxr,bozzmob/dxr,srenatus/dxr,jay-z007/dxr,jbradberry/dxr,jbradberry/dxr,erikrose/dxr,erikrose/dxr,nrc/dxr,gartung/dxr,pombredanne/dxr,kleintom/dxr,pelmers/dxr,bozzmob/dxr,bozzmob/dxr,pelmers/dxr,srenatus/dxr,pelmers/dxr,jbradberry/dxr,jbradberry/dxr,KiemVM/Mozilla--dxr,jonasfj/dxr,gartung/dxr,kleintom/dxr,jay-z007/dxr,gartung/dxr,jay-z007/dxr,jay-z007/dxr,KiemVM/Mozilla--dxr,jay-z007/dxr,jonasfj/dxr,kleintom/dxr,srenatus/dxr,bozzmob/dxr,KiemVM/Mozilla--dxr