Revert "lavu/timer: remove gratuitous volatile"

It does not make much sense to me, but GCC somehow optimises the
inline assembler even though the output is very obviously used and
having observable side effects.

This reverts commit 09731fbfc3.

libavutil/riscv/timer.h

@@ -29,13 +29,13 @@ static inline uint64_t ff_read_time(void)
 #if (__riscv_xlen >= 64)
     uintptr_t cycles;
 
-    __asm__ ("rdtime  %0" : "=r" (cycles));
+    __asm__ volatile ("rdtime  %0" : "=r" (cycles));
 
 #else
     uint64_t cycles;
     uint32_t hi, lo, check;
 
-    __asm__ (
+    __asm__ volatile (
         "1: rdtimeh %0\n"
         "   rdtime  %1\n"
         "   rdtimeh %2\n"