Add "Q_UNUSED" to (3rdparty) strtod.cc for a 32 bit code path

Accidentally stepped on this thing. Cannot build Qt Core on 32-bit
Ubuntu 16 without this patch.

Fixes: QTBUG-90354
Pick-to: 6.0 5.15
Change-Id: Iab5b2a317ee18b537b416eff008db29932fc043b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/3rdparty/double-conversion/patches/0001-Fix-unused-parameter-compiler-warning-on-32-bit-Linux.patch

@@ -0,0 +1,17 @@
+diff --git a/src/3rdparty/double-conversion/strtod.cc b/src/3rdparty/double-conversion/strtod.cc
+index e8cc13f2de..1b9f0f5b74 100644
+--- a/src/3rdparty/double-conversion/strtod.cc
++++ b/src/3rdparty/double-conversion/strtod.cc
+@@ -198,6 +198,12 @@ static bool DoubleStrtod(Vector<const char> trimmed,
+                          int exponent,
+                          double* result) {
+ #if !defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
++  // NB: Qt uses -Werror=unused-parameter which results in compiler error here
++  //     in this branch. Using "(void)x" idiom to prevent the error.
++  (void)trimmed;
++  (void)exponent;
++  (void)result;
++
+   // On x86 the floating-point stack can be 64 or 80 bits wide. If it is
+   // 80 bits wide (as is the case on Linux) then double-rounding occurs and the
+   // result is not accurate.

src/3rdparty/double-conversion/strtod.cc

@@ -198,6 +198,12 @@ static bool DoubleStrtod(Vector<const char> trimmed,
                          int exponent,
                          double* result) {
 #if !defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS)
+  // NB: Qt uses -Werror=unused-parameter which results in compiler error here
+  //     in this branch. Using "(void)x" idiom to prevent the error.
+  (void)trimmed;
+  (void)exponent;
+  (void)result;
+
   // On x86 the floating-point stack can be 64 or 80 bits wide. If it is
   // 80 bits wide (as is the case on Linux) then double-rounding occurs and the
   // result is not accurate.