More portable code for sockets

[XVilka]

See these errors due to missing <sys/select.h> while compiling on WIndows + MINGW32:

make[2]: Entering directory `/d/Work/radare/radare2/shlr/gdb'
mkdir -p lib
gcc -c  -MD    -g -Wall -DMINGW32=1 -I/d/Work/radare/radare2/libr/include -Iincl
ude -o src/core.o src/core.c
gcc -c  -MD    -g -Wall -DMINGW32=1 -I/d/Work/radare/radare2/libr/include -Iincl
ude -o src/libgdbr.o src/libgdbr.c
gcc -c  -MD    -g -Wall -DMINGW32=1 -I/d/Work/radare/radare2/libr/include -Iincl
ude -o src/messages.o src/messages.c
gcc -c  -MD    -g -Wall -DMINGW32=1 -I/d/Work/radare/radare2/libr/include -Iincl
ude -o src/packet.o src/packet.c
src/packet.c: In function 'send_packet':
src/packet.c:103:2: warning: implicit declaration of function 'send' [-Wimplicit
-function-declaration]
src/packet.c: In function 'read_packet':
src/packet.c:109:2: error: unknown type name 'fd_set'
src/packet.c:111:17: error: storage size of 'tv' isn't known
src/packet.c:119:3: warning: implicit declaration of function 'FD_ZERO' [-Wimpli
cit-function-declaration]
src/packet.c:120:3: warning: implicit declaration of function 'FD_SET' [-Wimplic
it-function-declaration]
src/packet.c:121:3: warning: implicit declaration of function 'select' [-Wimplic
it-function-declaration]
src/packet.c:123:4: warning: implicit declaration of function 'FD_ISSET' [-Wimpl
icit-function-declaration]
src/packet.c:124:5: warning: implicit declaration of function 'recv' [-Wimplicit
-function-declaration]
src/packet.c:111:17: warning: unused variable 'tv' [-Wunused-variable]
make[2]: *** [src/packet.o] Error 1
make[2]: Leaving directory `/d/Work/radare/radare2/shlr/gdb'
make[1]: *** [libgdbr] Error 2
make[1]: Leaving directory `/d/Work/radare/radare2/shlr'
make: *** [all] Error 2