make sure to include alloca.h before using it

[benpicco]

We have to include alloca.h if we want to use the alloca() function.

build/pkg/mjson/src/mjson.c: In function ‘mjson_merge’:
build/pkg/mjson/src/mjson.c:823:27: error: implicit declaration of function ‘alloca’ [-Werror=implicit-function-declaration]
     char *path = (char *) alloca((size_t) klen + 1);
                           ^
cc1: all warnings being treated as errors