includes: Move *abs() declarations into stdlib.h

Change-Id: Id4df2d3210735bee737353d293450e59cf93bd9a
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55593
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/stdlib.h

@@ -201,6 +201,15 @@ int rand(void);
 void srand(unsigned int seed);
 /** @} */
 
+/**
+ * @defgroup misc Misc functions
+ * @{
+ */
+int abs(int j);
+long int labs(long int j);
+long long int llabs(long long int j);
+/** @} */
+
 /* Enter remote GDB mode. Will initialize connection if not already up. */
 void gdb_enter(void);
 /* Disconnect existing GDB connection if one exists. */