Try this:

in unit1.h:

extern xdata char str1 [];

This provides a declaration of str1 - it tells the Compiler what str1 is (an array of characters in XDATA) without actually creating any code or data;

in unit1.c:

#include "unit1.h"

:

xdata char str1 [8];

This provides the definition of str1 - this is what actually creates the object in XDATA.
Note that I have included "unit1.h" here - this allows the compiler to report an error if your declaration of str1 in the header does not match the definition in unit1.c

in unit2.c:

#include "unit1.h"

:

void func ( void )
{ 
    int a; 
    a = atoi(str1); 
}

Here, we include the header file - rather than trying to re-type the declaration with the attendant risk of not (quite) matching the definition
(which is the root of your problems here).

BTW: Note also the use of the

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