entropy: Avoid volatile post-increment under C++20

C++20 deprecated pre- and post-increment on volatile lvalues, causing
the JITTER_APT FIPS break test path in MemoryAccessSampler::Next to fail
compilation with -Wdeprecated-volatile (Clang) and -Wvolatile (GCC) when
building BoringSSL consumers as C++20.

Use compound assignment (which C++20 defect report P2327R1 un-deprecated
when its result is discarded) to match the existing volatile memory
access in the same function.

Bug: b/333039464, b/491765858
Change-Id: I2e233ce91c922441d744fb8ac084b9c35c4250e6
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/103828
Auto-Submit: Tom Hughes <tomhughes@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>

crypto/fipsmodule/entropy/jitter.cc.inc

diff --git a/crypto/fipsmodule/entropy/jitter.cc.inc b/crypto/fipsmodule/entropy/jitter.cc.inc
index f4767a8..8c75166 100644
--- a/crypto/fipsmodule/entropy/jitter.cc.inc
+++ b/crypto/fipsmodule/entropy/jitter.cc.inc
@@ -119,7 +119,7 @@
       // processed by `DeltaSampler`, the tests see 0,0,0,1,... which exceeds
       // the APT cutoff (348/512) without tripping the RCT (26).
       *out = array_[0] >> 2;
-      array_[0]++;
+      array_[0] += 1;
     } else {
       // Perform some memory accesses and measure how long it took. The LCG is
       // intended to defeat any CPU predictors and thus expose this code to as