iceberg: add the example program

Port examples/iceberg.c from the source tree: a full Iceberg session
demonstrating the call order from the module docs -- distributed key
generation, pubshare_gen/pubkey_agg to obtain the group public key,
nonce_gen/nonce_agg into an ordinary MuSig2 public nonce, and
partial_sign/partial_sig_agg into an ordinary MuSig2 partial
signature.

One content adaptation: the secp256k1_musig_nonce_process call gains
a NULL adaptor argument, matching this repo's zkp musig variant.

Wired like the chilldkg example: autotools noinst_PROGRAMS +
TESTS entry under ENABLE_MODULE_ICEBERG (the example runs as part of
make check), CMake example target in examples/CMakeLists.txt, and
iceberg_example added to .gitignore.

Verified: ./iceberg_example runs to completion (exit 0) under both
build systems.

Makefile.am

@@ -234,6 +234,17 @@ chilldkg_example_LDFLAGS += -lbcrypt
 endif
 TESTS += chilldkg_example
 endif
+if ENABLE_MODULE_ICEBERG
+noinst_PROGRAMS += iceberg_example
+iceberg_example_SOURCES = examples/iceberg.c
+iceberg_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
+iceberg_example_LDADD = libsecp256k1.la
+iceberg_example_LDFLAGS = -static
+if BUILD_WINDOWS
+iceberg_example_LDFLAGS += -lbcrypt
+endif
+TESTS += iceberg_example
+endif
 endif
 
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