Merge bitcoin/bitcoin#30216: build: Fix building fuzz binary on on SunOS / illumos

3299abc build: Fix building `fuzz` binary on on SunOS / illumos (Hennadii Stepanov)

Pull request description:

  On master branch @ 457e184, building the `fuzz` binary fails:
  ```
  $ ./autogen.sh
  $ ./configure
  $ gmake -C src test/fuzz/fuzz
  < snip >
    CXX      test/fuzz/fuzz-http_request.o
  test/fuzz/http_request.cpp:13:10: fatal error: event2/buffer.h: No such file or directory
     13 | #include <event2/buffer.h>
        |          ^~~~~~~~~~~~~~~~~
  compilation terminated.
  gmake: *** [Makefile:17138: test/fuzz/fuzz-http_request.o] Error 1
  gmake: Leaving directory '/export/home/hebasto/git/bitcoin/src'
  ```

  The testing system:
  ```
  $ uname -a
  SunOS openindiana 5.11 illumos-82079dec87 i86pc i386 i86pc
  ```

  This PR fixes this issue.

ACKs for top commit:
  maflcko:
    ACK 3299abc

Tree-SHA512: 43048cf0d3db47d71263da179e07225afd901ed2039ee4d17314ff7b581ab36f41282fde3b1210926cecda546320dc573937c564520f61fbb236c2b9914ed0d4

Author: fanquake

src/Makefile.test.include

@@ -277,7 +277,7 @@ FUZZ_SUITE_LD_COMMON += $(LIBBITCOIN_ZMQ) $(ZMQ_LIBS)
 endif
 
 if ENABLE_FUZZ_BINARY
-test_fuzz_fuzz_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS)
+test_fuzz_fuzz_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS) $(EVENT_CFLAGS)
 test_fuzz_fuzz_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 test_fuzz_fuzz_LDADD = $(FUZZ_SUITE_LD_COMMON)
 test_fuzz_fuzz_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS) $(PTHREAD_FLAGS)