fix: use units in select label padding calculation (#5848)

Addition in css' `calc` function
[requires both operands to be `<length>`](https://css-tricks.com/a-complete-guide-to-calc-in-css/#aa-addition-and-subtraction-require-both-numbers-to-be-lengths)

packages/primevue/src/select/style/SelectStyle.js

@@ -83,7 +83,7 @@ const theme = ({ dt }) => `
 }
 
 .p-select:has(.p-select-clear-icon) .p-select-label {
-    padding-right: calc(1 + ${dt('select.padding.x')});
+    padding-right: calc(1rem + ${dt('select.padding.x')});
 }
 
 .p-select.p-disabled .p-select-label {