Allow the public header to work in C++ mode.

Without this, includes of the header must be wrapped in `extern "C"`,
which then causes problems if the config header includes headers that
expect to be included in C or C++ mode.

Signed-off-by: David Chisnall <David.Chisnall@scisemi.com>

Author: David Chisnall

.clang-format

@@ -17,6 +17,7 @@ IncludeBlocks: Preserve
 # as "attributes" so they don't get increasingly indented line after line
 BreakBeforeBraces: Allman
 InsertBraces: true
+IndentExternBlock: NoIndent
 WhitespaceSensitiveMacros: ['__contract__', '__loop__', 'MLK_RV64V_ABS_BOUNDS16' ]
 Macros:
  # Make this artifically long to avoid function bodies after short contracts

mlkem/mlkem_native.h

@@ -183,6 +183,11 @@
 
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
 /*************************************************
  * Name:        crypto_kem_keypair_derand
  *
@@ -377,6 +382,10 @@ MLK_API_MUST_CHECK_RETURN_VALUE
 int MLK_API_NAMESPACE(check_sk)(
     const uint8_t sk[MLKEM_SECRETKEYBYTES(MLK_CONFIG_API_PARAMETER_SET)]);
 
+#ifdef __cplusplus
+}
+#endif
+
 /****************************** SUPERCOP API *********************************/
 
 #if !defined(MLK_CONFIG_API_NO_SUPERCOP)