build-pkg: replace _ with . in versions

Underscores in versions were previously replaced with dashes.
Example: 4_1_3 -> 4-1-3 (package winpcap).
"-3" is interpreted as Debian patch version, which is wrong.

tools/build-pkg.lua

@@ -382,7 +382,7 @@ local function nameToDebian(item)
 end
 
 local function protectVersion(ver)
-    ver = ver:gsub('_', '-')
+    ver = ver:gsub('_', '.')
     if ver:sub(1, 1):match('%d') then
         return ver
     else