configure: Fix _GNU_SOURCE redefinition warning

```
gcc -D_GNU_SOURCE -include config-host.h -Werror=implicit-function-declaration \
-o /tmp/tmp.UrAfu1BQqA/liburing-conf.exe /tmp/tmp.UrAfu1BQqA/liburing-conf.c

/tmp/tmp.UrAfu1BQqA/liburing-conf.c:1: warning: "_GNU_SOURCE" redefined
    1 | #define _GNU_SOURCE
      |
<command-line>: note: this is the location of the previous definition
```

Introduced by commit 17d25f4c73b89 ("test/io_uring_register: fix
-Wimplicit-function-declaration of memfd_create").

Fixes: 17d25f4c73b89dfc722a367914776577e266638c ("test/io_uring_register: fix -Wimplicit-function-declaration of memfd_create")
Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>

1 files changed, 0 insertions, 1 deletions

diff --git a/configure b/configure
index 1f7f1c3..b8ecb7b 100755
--- a/configure
+++ b/configure
@@ -369,7 +369,6 @@ print_config "has_ucontext" "$has_ucontext"
 # check for memfd_create(2)
 has_memfd_create="no"
 cat > $TMPC << EOF
-#define _GNU_SOURCE
 #include <sys/mman.h>
 int main(int argc, char **argv)
 {