Normalize extern "C" guards to #ifdef __cplusplus (ros2#585)

char_array.h, uint8_array.h, and time.c used `#if __cplusplus`, while all
other headers (~33 sites) use `#ifdef __cplusplus`. Align the three outliers
to the dominant convention, avoiding the -Wundef diagnostics these guards
triggered. No behavioral change.

Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>

Author: zacheryasc

include/rcutils/types/char_array.h

@@ -17,7 +17,7 @@
 #ifndef RCUTILS__TYPES__CHAR_ARRAY_H_
 #define RCUTILS__TYPES__CHAR_ARRAY_H_
 
-#if __cplusplus
+#ifdef __cplusplus
 extern "C"
 {
 #endif
@@ -239,7 +239,7 @@ RCUTILS_WARN_UNUSED
 rcutils_ret_t
 rcutils_char_array_strcpy(rcutils_char_array_t * char_array, const char * src);
 
-#if __cplusplus
+#ifdef __cplusplus
 }
 #endif
 

include/rcutils/types/uint8_array.h

@@ -17,7 +17,7 @@
 #ifndef RCUTILS__TYPES__UINT8_ARRAY_H_
 #define RCUTILS__TYPES__UINT8_ARRAY_H_
 
-#if __cplusplus
+#ifdef __cplusplus
 extern "C"
 {
 #endif
@@ -114,7 +114,7 @@ RCUTILS_WARN_UNUSED
 rcutils_ret_t
 rcutils_uint8_array_resize(rcutils_uint8_array_t * uint8_array, size_t new_size);
 
-#if __cplusplus
+#ifdef __cplusplus
 }
 #endif
 

src/time.c

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#if __cplusplus
+#ifdef __cplusplus
 extern "C"
 {
 #endif
@@ -123,6 +123,6 @@ rcutils_time_point_value_as_seconds_string(
   return RCUTILS_RET_OK;
 }
 
-#if __cplusplus
+#ifdef __cplusplus
 }
 #endif