innosetup: move GPL license RTF to build-extra

Move the GPL RTF license file from the InnoSetup `installer` directory to
the top-level `build-extra` directory since it is a "shared resource" that
will be used by both the InnoSetup and MSI installers.

Signed-off-by: Craig E. Shea <craig.e.shea@gmail.com>

installer/install.iss

@@ -57,7 +57,7 @@ DisableDirPage=auto
 DefaultGroupName={#APP_NAME}
 DisableProgramGroupPage=auto
 DisableReadyPage=yes
-InfoBeforeFile={#SourcePath}\gpl-2.0.rtf
+InfoBeforeFile={#SourcePath}\..\gpl-2.0.rtf
 #ifdef OUTPUT_TO_TEMP
 PrivilegesRequired=lowest
 #else