Update linux webview positon fallback values values to match type declarations

[ttytm]

Fixes a warning when compiling on Linux caused by the webview struct taking uints but passing a negative value:

src/webui.c: In function ‘_webui_wv_show’:
src/webui.c:9728:45: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
 9728 |         webView->x = (win->x > 0 ? win->x : -1);
      |                                             ^~
src/webui.c:9729:45: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
 9729 |         webView->y = (win->y > 0 ? win->y : -1);
      |                                             ^~