evm: the Metal anchor's C half carries the same build tag as its Go half

The Go side of the secp256k1 Metal anchor is built only under
cgo && lux_cevm_native && darwin. Its C file carried no constraint at all, so
every other build still saw a C source in a package it was compiling without
cgo, and refused: "C source files not allowed when not using cgo or SWIG".
That took out `go build ./...` for the whole repository, not just this package.

A file and its other half now agree about when they exist. Under the tag the
anchor is still compiled in — go list reports it in CFiles — and without it,
neither half is.

Co-authored-by: Hanzo Dev <[email protected]>

evm/cevm/cevm_secp256k1_metal_anchor_darwin.c

@@ -4,6 +4,8 @@
 // Anchor for the secp256k1 Metal driver symbol. See cpp/ecrecover.cpp in
 // luxcpp/crypto for the dispatch logic that resolves this symbol via dlsym.
 
+//go:build cgo && lux_cevm_native && darwin
+
 #include <stddef.h>
 #include <stdint.h>