libswscale/riscv: fix syntax of vsetvli

Add missing operand which clang complains about but GCC assumes it to be
'm1' if not specified.

Works around build failure with Clang:
| src/libswscale/riscv/rgb2rgb_rvv.S:88:25: error: operand must be e[8|16|32|64|128|256|512|1024],m[1|2|4|8|f2|f4|f8],[ta|tu],[ma|mu]
|         vsetvli t4, t3, e8, ta, ma
|                         ^

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

libswscale/riscv/rgb2rgb_rvv.S

@@ -85,7 +85,7 @@ func ff_interleave_bytes_rvv, zve32x
         mv      t3, a3
         addi    a4, a4, -1
 2:
-        vsetvli    t4, t3, e8, ta, ma
+        vsetvli    t4, t3, e8, m1, ta, ma
         sub        t3, t3, t4
         vle8.v     v8, (t0)
         add        t0, t4, t0