chore: add SPDX headers to source files

Author: hyperpolymath

src/abi/Foreign.idr

@@ -1,3 +1,4 @@
+-- SPDX-License-Identifier: PMPL-1.0-or-later
 ||| Foreign Function Interface Declarations
 |||
 ||| This module declares all C-compatible functions that will be

src/abi/Layout.idr

@@ -1,3 +1,4 @@
+-- SPDX-License-Identifier: PMPL-1.0-or-later
 ||| Memory Layout Proofs
 |||
 ||| This module provides formal proofs about memory layout, alignment,

src/abi/Types.idr

@@ -1,3 +1,4 @@
+-- SPDX-License-Identifier: PMPL-1.0-or-later
 ||| ABI Type Definitions Template
 |||
 ||| This module defines the Application Binary Interface (ABI) for this library.