Fix error: static declaration of copy_file_range follows non-static d…

[remicollet]

On RHEL-8 where copy_file_range exists but kernel is too old

/builddir/build/BUILD/php-8.6.0alpha3/main/io/php_io_copy_linux.c:24:23: error: static declaration of 'copy_file_range' follows non-static declaration
 static inline ssize_t copy_file_range(
                       ^~~~~~~~~~~~~~~
In file included from /builddir/build/BUILD/php-8.6.0alpha3/Zend/zend_hrtime.h:23,
                 from /builddir/build/BUILD/php-8.6.0alpha3/Zend/zend_gc.h:22,
                 from /builddir/build/BUILD/php-8.6.0alpha3/Zend/zend_string.h:22,
                 from /builddir/build/BUILD/php-8.6.0alpha3/Zend/zend.h:31,
                 from /builddir/build/BUILD/php-8.6.0alpha3/main/php.h:28,
                 from /builddir/build/BUILD/php-8.6.0alpha3/main/php_io.h:20,
                 from /builddir/build/BUILD/php-8.6.0alpha3/main/io/php_io_internal.h:18,
                 from /builddir/build/BUILD/php-8.6.0alpha3/main/io/php_io_copy_linux.c:17:
/usr/include/unistd.h:1107:9: note: previous declaration of 'copy_file_range' was here
 ssize_t copy_file_range (int __infd, __off64_t *__pinoff,
         ^~~~~~~~~~~~~~~