[FIX] web: add missing gap between PDF buttons

=== ISSUE ===

In some modules, there is a recurring layout issue with buttons being
compressed. This is caused by a missing `gap` or `margin` and
results in two main issues :

1/ It reduces the clickable zone of each button (you are more likely
to click on its neighbor rather than the one you wanted)

2/ It does not look good and causes inconsistency with other buttons.

=== AFTER ===

We simply add a `d-flex` and a `gap-1` class to the container to be sure
that no matter how many buttons are inside that <div> they will
always be spaced correctly.

task-3433602
part of task-3326263

closes odoo/odoo#129906

X-original-commit: 6fc57407478e79232e46c21e21981eeff419d884
Signed-off-by: Pierre Paridans (app) <app@odoo.com>

addons/web/static/src/views/fields/pdf_viewer/pdf_viewer_field.xml

@@ -3,7 +3,7 @@
 
     <t t-name="web.PdfViewerField" owl="1">
         <t t-if="!props.readonly">
-            <div  class="o_form_pdf_controls">
+            <div class="o_form_pdf_controls d-flex gap-1">
                 <t t-if="props.record.data[props.name]">
                     <FileUploader
                         acceptedFileExtensions="'.pdf'"