Guard _DEFAULT_SOURCE with #ifndef in src/api.c (#422)

Guard the definition of _DEFAULT_SOURCE in src/api.c with #ifndef to prevent macro redefinition build errors when _DEFAULT_SOURCE or _GNU_SOURCE is already defined by the build environment (e.g. -D_GNU_SOURCE=1 or -D_DEFAULT_SOURCE=1).

src/api.c

diff --git a/src/api.c b/src/api.c
index 846235a..2b2233d 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1,5 +1,7 @@
 /* for syscall() */
+#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE
+#endif
 
 #include <stdbool.h>
 #include <stddef.h>