networking/test/chksum.c: fix undefined-function error.

include/networking.h defines __chksum_arm_simd conditionally, under
`#if (__arm__ && __ARM_NEON)`. But the test program refers to it under
just `#if __arm__`. So if you have __arm__ but not __ARM_NEON, you get
the reference, but not the definition to go with it, causing a link
failure.

Fixed by making the `#if`s in the test match the ones in the header.

networking/test/chksum.c

@@ -74,9 +74,9 @@ static struct
 {
     { checksum_simple, "simple"},
     { __chksum, "scalar"},
-#if __arm__
+#if __arm__ && __ARM_NEON
     { __chksum_arm_simd, "simd" },
-#elif __aarch64__
+#elif __aarch64__ && __ARM_NEON
     { __chksum_aarch64_simd, "simd" },
 #endif
     { NULL, NULL}