Allow building static binaries

sqlite uses CGO which prevents the building of statically-linked binaries.
This change will omit sqlite support when building a static binary with:
CGO_ENABLED=0 go build --ldflags '-extldflags "-static"'

server/store/sqldb.go

@@ -10,7 +10,6 @@ import (
 	"golang.org/x/crypto/ssh"
 
 	"github.com/go-sql-driver/mysql"
-	_ "github.com/mattn/go-sqlite3" // required by sql driver
 	"github.com/nsheridan/cashier/server/config"
 )
 

server/store/sqlite.go

+// +build cgo
+
+package store
+
+import _ "github.com/mattn/go-sqlite3"