fuzzers: use system includes

Use the system includes (defined by libgit2) as the fuzzer includes.
The fuzzers link against internal libgit2 API and therefore need to have
the full include path that libgit2 uses.

fuzzers/CMakeLists.txt

 LINK_DIRECTORIES(${LIBGIT2_LIBDIRS})
 INCLUDE_DIRECTORIES(${LIBGIT2_INCLUDES})
+INCLUDE_DIRECTORIES(SYSTEM ${LIBGIT2_SYSTEM_INCLUDES})
 
 IF(BUILD_FUZZERS AND NOT USE_STANDALONE_FUZZERS)
 	ADD_C_FLAG(-fsanitize=fuzzer)