pld: make get_pld_device_by_num() static

The function is not referenced outside the file.
Make it static.

Change-Id: I5f2a2c70085b9158df8806432bb9ed09bb256ab5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8546
Tested-by: jenkins

src/pld/pld.c

@@ -28,7 +28,7 @@ static struct pld_driver *pld_drivers[] = {
 
 static struct pld_device *pld_devices;
 
-struct pld_device *get_pld_device_by_num(int num)
+static struct pld_device *get_pld_device_by_num(int num)
 {
 	struct pld_device *p;
 	int i = 0;