Add 'includes' to CompileSpec

The includes field is an array of regexp used to indicate what include
inside the packag. It can be omitted to default behavior (all)

pkg/compiler/spec.go

@@ -31,6 +31,7 @@ type LuetCompilationSpec struct {
 	Package    pkg.Package `json:"-"`
 	OutputPath string      `json:"-"` // Where the build processfiles go
 	Unpack     bool        `json:"unpack"`
+	Includes   []string    `json:"includes"`
 }
 
 func NewLuetCompilationSpec(b []byte, p pkg.Package) (CompilationSpec, error) {
@@ -59,6 +60,10 @@ func (cs *LuetCompilationSpec) GetPreBuildSteps() []string {
 	return cs.Prelude
 }
 
+func (cs *LuetCompilationSpec) GetIncludes() []string {
+	return cs.Includes
+}
+
 func (cs *LuetCompilationSpec) GetSeedImage() string {
 	return cs.Seed
 }