scripts/checkpatch: Improve blank line check after declarations

This adds volatile type qualifier to the struct declaration matching
when looking for blank line after declarations.

Before this commit, this structure was not accepted by chheckpatch:
  struct cfg {
         struct gpio_driver_config common;
         volatile struct grgpio_regs *regs;
         int interrupt;
  };

checkpatch.pl generated the following warning:
  -:158: WARNING:LINE_SPACING: Missing a blank line after declarations
  #158: FILE: drivers/gpio/gpio_grgpio2.c:27:
  +	struct gpio_driver_config common;
  +	volatile struct grgpio_regs *regs;

With this commit, the warning is no longer generated.

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>