[gdb/contrib] Fix "sofar->so far" misspelling

I forgot to follow up on a review comment and fix the "sofar->so far"
misspelling [1].

Fix this by adding it to gdb/contrib/common-misspellings.txt.

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2024-September/211894.html

gdb/contrib/common-misspellings.txt

@@ -15,3 +15,4 @@
 # Common spelling mistakes.
 
 inbetween->between, in between, in-between
+sofar->so far

gdb/run-on-main-thread.c

@@ -85,7 +85,7 @@ run_events (int error, gdb_client_data client_data)
 	{
 	  /* Should cancelation of a runnable event cancel the execution of
 	     the following one?  The answer is not clear, so keep doing what
-	     we've done sofar: ignore this exception.  */
+	     we've done so far: ignore this exception.  */
 	}
       catch (const gdb_exception &)
 	{

gdb/symtab.c

@@ -2992,7 +2992,7 @@ find_pc_sect_compunit_symtab (CORE_ADDR pc, struct obj_section *section)
 					   section.  */
 	    }
 
-	  /* Cust is best found sofar, save it.  */
+	  /* Cust is best found so far, save it.  */
 	  best_cust = cust;
 	  best_cust_range = range;
 	}