x86: Fix test-double-vlen include-next

clang issues:

  ../sysdeps/x86_64/fpu/test-double-vlen4.h:20:2: error: #include_next in
  file found relative to primary source file or found by absolute path;
  will search from start of include path
  [-Werror,-Winclude-next-absolute-path]

Instead of suppress the warning, it is simpler to just include the
absolute path instead.

sysdeps/x86_64/fpu/test-double-vlen4.h

@@ -16,6 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include_next <test-double-vlen4.h>
+#include <math/test-double-vlen4.h>
 
 #define REQUIRE_AVX

sysdeps/x86_64/fpu/test-double-vlen8.h

@@ -16,6 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include_next <test-double-vlen8.h>
+#include <math/test-double-vlen8.h>
 
 #define REQUIRE_AVX512F

sysdeps/x86_64/fpu/test-float-vlen16.h

@@ -16,6 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include_next <test-float-vlen16.h>
+#include <math/test-float-vlen16.h>
 
 #define REQUIRE_AVX512F

sysdeps/x86_64/fpu/test-float-vlen8.h

@@ -16,6 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include_next <test-float-vlen8.h>
+#include <math/test-float-vlen8.h>
 
 #define REQUIRE_AVX