RVV: add missing SPDX-License-Identifier to PacketMathDecl.h

Rasmus Munk Larsenrequested to merge
rmlarsen1/eigen-new:rvv10-decl-spdx into master

The file was added in !2502 with the standard MPL2 copyright comment but without the SPDX-License-Identifier: MPL-2.0 tag, so reuse lint (run in the checkformat:reuse CI job) flags it as missing licensing information. Sibling RVV10 headers (PacketMathBF16.h, PacketMathFP16.h) already carry the tag; this just brings the new file to parity.

cc @cantonios

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