[Patch] comctl32: fix signedness compilation warning

When compiling with clang the following earnings are generated:

../../../dlls/comctl32/taskdialog.c:323:24: warning: taking the  
absolute value of unsigned type 'unsigned int' has no effect  
[-Wabsolute-value]
         int diff_now = abs(line_widths[i] - line_widths[i - 1]);
                        ^
../../../dlls/comctl32/taskdialog.c:323:24: note: remove the call to  
'abs' since unsigned values cannot be negative
         int diff_now = abs(line_widths[i] - line_widths[i - 1]);
                        ^~~
../../../dlls/comctl32/taskdialog.c:332:24: warning: taking the  
absolute value of unsigned type 'unsigned int' has no effect  
[-Wabsolute-value]
         diff_changed = abs(2 * buttons[last_button].width +  
line_widths[i] - line_widths[i - 1]);
                        ^
../../../dlls/comctl32/taskdialog.c:332:24: note: remove the call to  
'abs' since unsigned values cannot be negative
         diff_changed = abs(2 * buttons[last_button].width +  
line_widths[i] - line_widths[i - 1]);
                        ^~~

The patch casts first operand to (int) making intent clean.

Signed-off-by: Vlad Bespalov