Build fix for libwebp on INTEGRITY

Enabled C99 for INTEGRITY to allow non-constant values for array size

Change-Id: I51978d26a9ec4798eb541dc0a03b633ac9eb426a
Reviewed-by: Nikola Velinov <nvelinov@ghs.com>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/3rdparty/libwebp.pri

@@ -115,6 +115,10 @@ android {
     INCLUDEPATH += $$NDK_ROOT/sources/android/cpufeatures
 }
 
+integrity {
+    QMAKE_CFLAGS += -c99
+}
+
 equals(QT_ARCH, arm)|equals(QT_ARCH, arm64) {
     SOURCES_FOR_NEON += \
         $$PWD/libwebp/src/dsp/dec_neon.c \