meson: fix build on arm64

Would fail like:

Checking if "compiler supports ARMv7/AArch64 NEON intrinsics" : links: NO
Checking if "compiler supports ARMv7/AArch64 NEON intrinsics with -mfpu=neon" : links: YES
Checking if "compiler supports AArch64 NEON intrinsics" : links: NO
Checking if "compiler supports AArch64 NEON intrinsics with -mfpu=neon" : links: NO
Message: Compiler does not support AArch64 NEON intrinsics
../silk/meson.build:28:45: ERROR: Unknown variable "have_arm_intrinsics_or_asm".

since commit 08088411.

meson.build

@@ -574,8 +574,8 @@ foreach l : lines
 endforeach
 
 subdir('include')
-subdir('silk')
 subdir('celt')
+subdir('silk')
 subdir('src')
 
 configure_file(output: 'config.h', configuration: opus_conf)