tray: dexc-tray version shows incorrect app name

[davecgh]

The dexc-tray --version output doesn't show the right name. It shows dexc instead of dexc-tray as expected. It's particularly noticeable in the showvers.sh script in the release repo which shows the version of all of the built binaries one after the other:

$ ./showvers.sh
dcrd version 2.0.1+release (Go version go1.22.3 windows/amd64)
dcrctl version 2.0.1+release (Go version go1.22.3 windows/amd64)
dcrwallet version 2.0.1+release (Go version go1.22.3 windows/amd64)
dexc version 0.6.3+release (Go version go1.22.3 windows/amd64)     <--- This is actually dexc --version
dexcctl version 0.6.3+release (Go version go1.22.3 windows/amd64)
dexcctl required RPC server version: 0.3.0
dcrlncli version 0.7.1+release (Go version go1.22.3 windows/amd64)

dcrlnd version 0.7.1+release (Go version go1.22.3 windows/amd64)
dexc version 0.6.3+release (Go version go1.22.3 windows/amd64)     <--- This is dexc-tray --version, but you can't tell because it's misnamed
politeiavoter version 1.4.0+release (Go version go1.22.3 windows/amd64)