Fix some openSUSE build warnings

Fix implicit definitions of functions

[  155s] I: Program is using implicit definitions of functions getting
[  155s]    pointers or implemented by macros. These functions need to use their
[  155s]    correct prototypes to allow correct argument passing on e.g. x86_64 .
[  155s]      - Implicit memory/string functions need #include <string.h>.
[  155s]      - Implicit *printf functions need #include <stdio.h>.
[  155s]      - Implicit *printf functions need #include <stdio.h>.
[  155s]      - Implicit *read* functions need #include <unistd.h>.
[  155s]      - Implicit *recv* functions need #include <sys/socket.h>.
[  155s] W: ploticus implicit-pointer-decl dbl2png.c:274
[  155s] W: ploticus implicit-pointer-decl png2dbl.c:353
[  155s]
[  155s] I: Program is likely to break with new gcc. Try -fno-strict-aliasing.
[  155s] W: ploticus strict-aliasing-punning read.c:134, 146
[  155s]
[  155s] I: Program is using implicit definitions of special functions.
[  155s]    these functions need to use their correct prototypes to allow
[  155s]    the lightweight buffer overflow checking to work.
[  155s]      - Implicit memory/string functions need #include <string.h>.
[  155s]      - Implicit *printf functions need #include <stdio.h>.
[  155s]      - Implicit *printf functions need #include <stdio.h>.
[  155s]      - Implicit *read* functions need #include <unistd.h>.
[  155s]      - Implicit *recv* functions need #include <sys/socket.h>.
[  155s] E: ploticus implicit-fortify-decl dbl2png.c:258, 267
[  155s] E: ploticus implicit-fortify-decl png2dbl.c:245, 272, 337, 346
[  155s] E: ploticus implicit-fortify-decl pngdbl.c:97, 278

Signed-off-by: Sandro Santilli <strk@keybit.net>