remove a compilation warnings for libtest and test3

------------ libtest ------------
    ./libtcc_test lib_path=..
    <string>:11: warning: implicit declaration of function 'printf'
    <string>:13: warning: implicit declaration of function 'add'
    ------------ test3 ------------
    tcctest.c:1982: warning: implicit declaration of function 'putchar'
    tcctest.c:2133: warning: implicit declaration of function 'strlen'

tcclib.h

@@ -39,6 +39,7 @@ int getchar(void);
 char *gets(char *s);
 int ungetc(int c, FILE *stream);
 int fflush(FILE *stream);
+int putchar (int c);
 
 int printf(const char *format, ...);
 int fprintf(FILE *stream, const char *format, ...);
@@ -64,6 +65,7 @@ void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);
 char *strdup(const char *s);
+size_t strlen(const char *s);
 
 /* dlfcn.h */
 #define RTLD_LAZY       0x001

tests/libtcc_test.c

@@ -16,6 +16,8 @@ int add(int a, int b)
 }
 
 char my_program[] =
+"#include <stdio.h> // printf()\n"
+"extern int add(int a, int b);\n"
 "int fib(int n)\n"
 "{\n"
 "    if (n <= 2)\n"