fix(libc): remove __Nonnull type specifier

Clang's nullability completeness checks were triggered after adding
the _Nonnull specifier to one function. Removing it prevents Clang
from flagging atexit() for missing a nullability specifier.

 include/lib/libc/stdlib.h:25:25: error: pointer is missing a
 nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified)
 [-Werror,-Wnullability-completeness]
    25 | extern int atexit(void (*func)(void));

This change ensures compliance with the C standard while preventing
unexpected build errors.

Change-Id: I2f881c55b36b692d22c3db22149c6402c32e8c3e
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/include/lib/libc/stdlib.h b/include/lib/libc/stdlib.h
index cf30ef3..71bb4ba 100644
--- a/include/lib/libc/stdlib.h
+++ b/include/lib/libc/stdlib.h
@@ -30,5 +30,5 @@
 long long strtoll(const char *nptr, char **endptr, int base);
 unsigned long long strtoull(const char *nptr, char **endptr, int base);
 void qsort(void *, size_t, size_t,
-           int (* _Nonnull)(const void *, const void *));
+           int (*)(const void *, const void *));
 #endif /* STDLIB_H */