libxslt: add missing stdlib.h include for getenv/strtol

Clang defaults to C23, which errors on implicit function declarations.
libxslt 1.1.34's extensions.c and libexslt/date.c call getenv/strtol
wi

libxslt: add missing stdlib.h include for getenv/strtol

Clang defaults to C23, which errors on implicit function declarations.
libxslt 1.1.34's extensions.c and libexslt/date.c call getenv/strtol
without including stdlib.h, breaking the bundled build on modern
toolchains (e.g. Xcode clang on macOS).

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