[gdb] Fix some typos

Run gdb/contrib/spellcheck.sh on directories gdb*.

Fix typo:
...
unkown -> unknown
...

Tested on x86_64-linux.

gdb/nat/linux-btrace.c

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 7ff12d6..0b820cc 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -1004,7 +1004,7 @@ linux_read_btrace (struct btrace_data *btrace,
       return linux_read_pt (&btrace->variant.pt, tinfo, type);
     }
 
-  internal_error (_("Unkown branch trace format."));
+  internal_error (_("Unknown branch trace format."));
 }
 
 /* See linux-btrace.h.  */

gdbsupport/btrace-common.cc

diff --git a/gdbsupport/btrace-common.cc b/gdbsupport/btrace-common.cc
index 2358a9f..b7f9493 100644
--- a/gdbsupport/btrace-common.cc
+++ b/gdbsupport/btrace-common.cc
@@ -81,7 +81,7 @@
       return;
     }
 
-  internal_error (_("Unkown branch trace format."));
+  internal_error (_("Unknown branch trace format."));
 }
 
 /* See btrace-common.h.  */
@@ -101,7 +101,7 @@
       return (variant.pt.size == 0);
     }
 
-  internal_error (_("Unkown branch trace format."));
+  internal_error (_("Unknown branch trace format."));
 }
 
 /* See btrace-common.h.  */
@@ -184,5 +184,5 @@
       return 0;
     }
 
-  internal_error (_("Unkown branch trace format."));
+  internal_error (_("Unknown branch trace format."));
 }