fixed undefined reference to `makedev'

[yasintuncel]

add #include <sys/sysmacros.h> library to cmux.c for using makedev function

otherwise the code give below error

make: Warning: File 'cmux.c' has modification time 9.8 s in the future
gcc -Wall    cmux.c   -o cmux
cmux.c: In function ‘make_nodes’:
cmux.c:232:12: warning: implicit declaration of function ‘makedev’; did you mean ‘make_nodes’? [-Wimplicit-function-declaration]
  232 |   device = makedev(major, minor);
      |            ^~~~~~~
      |            make_nodes
/usr/bin/ld: /tmp/cctCHk58.o: in function `make_nodes':
cmux.c:(.text+0x3bc): undefined reference to `makedev'
collect2: error: ld returned 1 exit status
make: *** [<builtin>: cmux] Error 1