cmake: use C90 standard for our fuzzing targets

Like all our other internal code, we want to force the use of C90 for
our fuzzers. Do so by setting the "C_STANDARD" property of our fuzzing
targets.

fuzzers/CMakeLists.txt

@@ -9,5 +9,6 @@ FOREACH(fuzz_target_src ${SRC_FUZZ})
 		LIST(APPEND ${fuzz_target_name}_SOURCES "standalone_driver.c")
 	ENDIF()
 	ADD_EXECUTABLE(${fuzz_target_name} ${${fuzz_target_name}_SOURCES})
+	SET_TARGET_PROPERTIES(${fuzz_target_name} PROPERTIES C_STANDARD 90)
 	TARGET_LINK_LIBRARIES(${fuzz_target_name} ${LIBGIT2_LIBS})
 ENDFOREACH()