Use 'require' with gnatmake_version_at_least

I found a couple of tests that check gnatmake_version_at_least using
"if" where "require" would be a little cleaner.  This patch converts
these.

gdb/testsuite/gdb.ada/mi_prot.exp

diff --git a/gdb/testsuite/gdb.ada/mi_prot.exp b/gdb/testsuite/gdb.ada/mi_prot.exp
index 42ca90d..35d2c17 100644
--- a/gdb/testsuite/gdb.ada/mi_prot.exp
+++ b/gdb/testsuite/gdb.ada/mi_prot.exp
@@ -16,10 +16,8 @@
 load_lib "ada.exp"
 
 require allow_ada_tests
+require {gnatmake_version_at_least 8}
 
-if { ![gnatmake_version_at_least 8] } {
-    return -1
-}
 standard_ada_testfile prot
 
 if {[gdb_compile_ada "${srcfile}" "${binfile}" executable \

gdb/testsuite/gdb.ada/rename_subscript_param.exp

diff --git a/gdb/testsuite/gdb.ada/rename_subscript_param.exp b/gdb/testsuite/gdb.ada/rename_subscript_param.exp
index df5c9b2..e3209bc 100644
--- a/gdb/testsuite/gdb.ada/rename_subscript_param.exp
+++ b/gdb/testsuite/gdb.ada/rename_subscript_param.exp
@@ -17,9 +17,7 @@
 
 require allow_ada_tests
 
-if { ![gnatmake_version_at_least 8] } {
-    return -1
-}
+require {gnatmake_version_at_least 8}
 
 standard_ada_testfile pb30_012