build: avoid ODR violation from different struct layout per TU

[jengelh]

When building with ASAN, the linker warns about struct declarations being different between TUs, which is undefined behavior.

[  1%] Linking CXX shared library build/lib/libwmime.so
/usr/bin/cmake -E cmake_link_script CMakeFiles/wmime.dir/link.txt --verbose=1
/usr/bin/c++ -fPIC -D_REENTRANT=1 -W -Wall -pedantic -Wpointer-arith
-Wold-style-cast -Wconversion -Wcast-align -Wno-long-long -fvisibility=hidden
-fvisibility-inlines-hidden -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3
-fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables
-fstack-clash-protection -Werror=return-type -flto=auto -g -O0 -ggdb3
-fsanitize=address,undefined -lasan -lubsan
-Wl,--dependency-file=CMakeFiles/wmime.dir/link.d -flto=auto -Wl,--as-needed
-Wl,--no-undefined -Wl,-z,now -lasan -lubsan -shared -Wl,-soname,libwmime.so.2
-o build/lib/libwmime.so.2.0.0 CMakeFiles/wmime.dir/src/vmime/address.cpp.o...

~/src/vmime/security/sasl/SASLContext.hpp:47:20:
warning: type ‘struct SASLContext’ violates the C++ One Definition Rule [-Wodr]
   47 | class VMIME_EXPORT SASLContext : public object, public enable_shared_from_this <SASLContext> {
~/src/vmime/security/sasl/SASLContext.hpp:47:20: note: a different type is defined in another translation unit
   47 | class VMIME_EXPORT SASLContext : public object, public enable_shared_from_this <SASLContext> {
~/src/vmime/security/sasl/SASLContext.hpp:123:15: note: the first difference of corresponding definitions is field ‘m_gsaslContext’
  123 |         void* m_gsaslContext;
~/src/vmime/security/sasl/SASLContext.hpp:121:16: note: a field of same name but different type is defined in another translation unit
  121 |         Gsasl* m_gsaslContext;