Make opus_cpu_capabilities() static.

The MSVC implementations already was, but the others were not.
This is an implementation detail that should not have external
 visibility.
The public interface is opus_select_arch().

celt/arm/armcpu.c

@@ -96,7 +96,7 @@ static OPUS_INLINE opus_uint32 opus_cpu_capabilities(void){
 /* Linux based */
 #include <stdio.h>
 
-opus_uint32 opus_cpu_capabilities(void)
+static opus_uint32 opus_cpu_capabilities(void)
 {
   opus_uint32 flags = 0;
   FILE *cpuinfo;
@@ -169,7 +169,7 @@ opus_uint32 opus_cpu_capabilities(void)
 #include <sys/types.h>
 #include <sys/sysctl.h>
 
-opus_uint32 opus_cpu_capabilities(void)
+static opus_uint32 opus_cpu_capabilities(void)
 {
   opus_uint32 flags = 0;