ob-C: Adjust pcase pattern for Emacs 24 compatibility

* lisp/ob-C.el (org-babel-C-header-to-C): Switch pcase patterns to
"`X" rather than "'X", which isn't supported until Emacs 25.

These were introduced by 38f87a26b (ob-C.el: Fix a number a
regressions related to table parameters, 2021-04-29).

Reported-by: Ihor Radchenko <yantar92@gmail.com>
Link: https://orgmode.org/list/87y2d2mqik.fsf@localhost

lisp/ob-C.el

@@ -481,11 +481,11 @@ specifying a variable with the name of the table."
   (let ((table (car head))
         (headers (cdr head))
         (typename (pcase type
-                    ('integerp "int")
-                    ('floatp "double")
-                    ('stringp (pcase org-babel-c-variant
-                                ((or 'c 'cpp) "const char*")
-                                ('d "string"))))))
+                    (`integerp "int")
+                    (`floatp "double")
+                    (`stringp (pcase org-babel-c-variant
+                                ((or `c `cpp) "const char*")
+                                (`d "string"))))))
     (concat
       (pcase org-babel-c-variant
 	((or `c `cpp)