This fixes a compiler warning:
../sway/extensions.c: In function ‘set_background’:
../sway/extensions.c:16:37: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
struct background_config *config = malloc(sizeof(struct background_config));
^
../sway/extensions.c:16:37: warning: incompatible implicit declaration of built-in function ‘malloc’
../sway/extensions.c:16:37: note: include ‘<stdlib.h>’ or provide a declaration of ‘malloc’
a99aa92bc6