headers: fail Build on missing native exports and assert MI/COM layouts (#2739)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

internal/headers/slc/slc.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"unsafe"
 
+	"github.com/prometheus-community/windows_exporter/internal/headers/lazyproc"
 	"golang.org/x/sys/windows"
 )
 
@@ -28,8 +29,14 @@ import (
 var (
 	slc                         = windows.NewLazySystemDLL("slc.dll")
 	procSLIsWindowsGenuineLocal = slc.NewProc("SLIsWindowsGenuineLocal")
+
+	load = lazyproc.Once(slc, procSLIsWindowsGenuineLocal)
 )
 
+// Load checks that slc.dll exports every function of this package. Call it in
+// Build: a missing export panics on the first call otherwise.
+func Load() error { return load() }
+
 // SL_GENUINE_STATE enumeration
 //
 // https://learn.microsoft.com/en-us/windows/win32/api/slpublic/ne-slpublic-sl_genuine_state