[clang-tidy] Switch C headers to C++ ones

Found using modernize-return-braced-init-list

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Context.cpp

@@ -31,9 +31,9 @@
 #include <sstream>
 #include <iomanip>
 #include <algorithm>
-#include <assert.h>
-#include <stdlib.h>
-#include <string.h>
+#include <cassert>
+#include <cstdlib>
+#include <cstring>
 #include <unistd.h>
 #include <FS.h>
 #include <Eval.h>
@@ -48,7 +48,7 @@
 #include <commit.h>
 #endif
 
-#include <stdio.h>
+#include <cstdio>
 #include <sys/ioctl.h>
 
 #ifdef SOLARIS