stm32f1: Use __always_inline on timer_read_time

Some older versions of gcc need the __always_inline directive in order
to inline timer_read_time.  Inlining that function is important for
performance on the stm32f1.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32f1/timer.c

@@ -50,7 +50,7 @@ timer_kick(void)
 static uint32_t timer_high;
 
 // Return the current time (in absolute clock ticks).
-inline uint32_t
+__always_inline uint32_t
 timer_read_time(void)
 {
     uint32_t th = readl(&timer_high);