staging: gpib: make global 'usec_diff' functions static

Trying to build both gpib_bitbang and lpvo_usb_gpib into the kernel
reveals a function that should have been static and is also duplicated:

x86_64-linux-ld: drivers/staging/gpib/lpvo_usb_gpib/lpvo_usb_gpib.o: in function `usec_diff':
lpvo_usb_gpib.c:(.text+0x23c0): multiple definition of `usec_diff'; drivers/staging/gpib/gpio/gpib_bitbang.o:gpib_bitbang.c:(.text+0x2470): first defined here

Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/gpib/gpio/gpib_bitbang.c

···
 	enum listener_function_state listener_state;
 };
 
-inline long usec_diff(struct timespec64 *a, struct timespec64 *b);
+static inline long usec_diff(struct timespec64 *a, struct timespec64 *b);
 static void bb_buffer_print(unsigned char *buffer, size_t length, int cmd, int eoi);
 static void set_data_lines(u8 byte);
 static u8 get_data_lines(void);

drivers/staging/gpib/lpvo_usb_gpib/lpvo_usb_gpib.c

···
  *		 (unix time in sec and NANOsec)
  */
 
-inline int usec_diff(struct timespec64 *a, struct timespec64 *b)
+static inline int usec_diff(struct timespec64 *a, struct timespec64 *b)
 {
 	return ((a->tv_sec - b->tv_sec) * 1000000 +
 		(a->tv_nsec - b->tv_nsec) / 1000);