headergen: install g++ in Dockerfile to enable line-profiler build

The HeaderGen Docker image previously installed only gcc, not g++.
HeaderGen's transitive dependency `line-profiler` has a Cython C++
extension; on platforms where no prebuilt wheel exists (e.g., arm64
Linux + Python 3.10), pip falls back to building from source, which
requires g++. The build then fails with:

    g++ -... -c line_profiler/_line_profiler.cpp -o ...
    error: command 'g++' failed: No such file or directory
    ERROR: Failed building wheel for line-profiler

This change adds g++ to the apt-get install line. The image now builds
cleanly on arm64 hosts as well as amd64.

(Builds may have appeared to succeed previously on hosts where a
line-profiler wheel was already cached from an earlier build with g++
present; fresh builds without the cache hit the source-compile path
and surface the issue.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bgoconnor

src/target_tools/headergen/Dockerfile

@@ -10,7 +10,7 @@ WORKDIR /app
 
 # Install dependencies
 RUN apt-get update \
-    && apt-get -y install git gcc
+    && apt-get -y install git gcc g++
 
 
 COPY requirements.txt /app/requirements.txt